Full Form1.cs code –

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace zombieShooter
{
    public partial class Form1 : Form
    {

        // start the variables

        bool goup; // this boolean will be used for the player to go up the screen
        bool godown; // this boolean will be used for the player to go down the screen
        bool goleft; // this boolean will be used for the player to go left to the screen
        bool goright; // this boolean will be used for the player to right to the screen
        string facing = "up"; // this string is called facing and it will be used to guide the bullets
        double playerHealth = 100; // this double vaiable is called player health
        int speed = 10; // this interget is for the speed of the player
        int ammo = 10; // this integer will hold the number of ammo the player has start of the game
        int zombieSpeed = 3; // this integer will hold the speed which the zombies move in the game
        int score = 0; // this integer will hold the score the player achieved through the game
        bool gameOver = false; // this boolean is false in the beginning and it will be used when the game is finished
        Random rnd = new Random(); // this is an instance of the random class we will use this to create a random number for this game

        // end of listing variables


        public Form1()
        {
            InitializeComponent();
        }

        private void keyisdown(object sender, KeyEventArgs e)
        {
            if (gameOver) return; // if game over is true then do nothing in this event

            // if the left key is pressed then do the following
            if (e.KeyCode == Keys.Left)
            {
                goleft = true; // change go left to true
                facing = "left"; //change facing to left
                player.Image = Properties.Resources.left; // change the player image to LEFT image

            }

            // end of left key selection

            // if the right key is pressed then do the following
            if (e.KeyCode == Keys.Right)
            {
                goright = true; // change go right to true
                facing = "right"; // change facing to right
                player.Image = Properties.Resources.right; // change the player image to right

            }
            // end of right key selection

            // if the up key is pressed then do the following
            if (e.KeyCode == Keys.Up)
            {
                facing = "up"; // change facing to up
                goup = true; // change go up to true
                player.Image = Properties.Resources.up; // change the player image to up

            }

            // end of up key selection

            // if the down key is pressed then do the following
            if (e.KeyCode == Keys.Down)
            {
                facing = "down"; // change facing to down
                godown = true; // change go down to true
                player.Image = Properties.Resources.down; //change the player image to down

            }
            // end of the down key selection

        }

        private void keyisup(object sender, KeyEventArgs e)
        {
            if (gameOver) return; // if game is over then do nothing in this event

            // below is the key up selection for the left key
            if (e.KeyCode == Keys.Left)
            {
                goleft = false; // change the go left boolean to false
            }

            // below is the key up selection for the right key
            if (e.KeyCode == Keys.Right)
            {
                goright = false; // change the go right boolean to false
            }
            // below is the key up selection for the up key
            if (e.KeyCode == Keys.Up)
            {
                goup = false; // change the go up boolean to false
            }
            // below is the key up selection for the down key
            if (e.KeyCode == Keys.Down)
            {
                godown = false; // change the go down boolean to false
            }

            //below is the key up selection for the space key
            if (e.KeyCode == Keys.Space && ammo > 0) // in this if statement we are checking if the space bar is up and ammo is more than 0
            {
                ammo--; // reduce ammo by 1 from the total number
                shoot(facing); // invoke the shoot function with the facing string inside it
                //facing will transfer up, down, left or right to the function and that will shoot the bullet that way. 

                if (ammo < 1) // if ammo is less than 1
                {
                    DropAmmo(); // invoke the drop ammo function
                }
            }
        }

        private void gameEngine(object sender, EventArgs e)
        {
            if (playerHealth > 1) // if player health is greater than 1
            {
                progressBar1.Value = Convert.ToInt32(playerHealth); // assign the progress bar to the player health integer
            }
            else
            {
                // if the player health is below 1
                player.Image = Properties.Resources.dead; // show the player dead image
                timer1.Stop(); // stop the timer
                gameOver = true; // change game over to true
            }

            label1.Text = "   Ammo:  " + ammo; // show the ammo amount on label 1
            label2.Text = "Kills: " + score; // show the total kills on the score

            // if the player health is less than 20
            if (playerHealth < 20)
            {
                progressBar1.ForeColor = System.Drawing.Color.Red; // change the progress bar colour to red. 
            }

            if (goleft && player.Left > 0)
            {
                player.Left -= speed;
                // if moving left is true AND pacman is 1 pixel more from the left 
                // then move the player to the LEFT
            }
            if (goright && player.Left + player.Width < 930)
            {
                player.Left += speed;
                // if moving RIGHT is true AND player left + player width is less than 930 pixels
                // then move the player to the RIGHT
            }
            if (goup && player.Top > 60)
            {
                player.Top -= speed;
                // if moving TOP is true AND player is 60 pixel more from the top 
                // then move the player to the UP
            }

            if (godown && player.Top + player.Height < 700)
            {
                player.Top += speed;
                // if moving DOWN is true AND player top + player height is less than 700 pixels
                // then move the player to the DOWN
            }

            // run the first for each loop below
            // X is a control and we will search for all controls in this loop
            foreach (Control x in this.Controls)
            {
                // if the X is a picture box and X has a tag AMMO

                if (x is PictureBox && x.Tag == "ammo")
                {
                    // check is X in hitting the player picture box

                    if (((PictureBox)x).Bounds.IntersectsWith(player.Bounds))
                    {
                        // once the player picks up the ammo

                        this.Controls.Remove(((PictureBox)x)); // remove the ammo picture box

                        ((PictureBox)x).Dispose(); // dispose the picture box completely from the program
                        ammo += 5; // add 5 ammo to the integer
                    }
                }

                // if the bullets hits the 4 borders of the game
                // if x is a picture box and x has the tag of bullet

                if (x is PictureBox && x.Tag == "bullet")
                {
                    // if the bullet is less the 1 pixel to the left
                    // if the bullet is more then 930 pixels to the right
                    // if the bullet is 10 pixels from the top
                    // if the bullet is 700 pixels to the buttom

                    if (((PictureBox)x).Left < 1 || ((PictureBox)x).Left > 930 || ((PictureBox)x).Top < 10 || ((PictureBox)x).Top > 700)
                    {
                        this.Controls.Remove(((PictureBox)x)); // remove the bullet from the display
                        ((PictureBox)x).Dispose(); // dispose the bullet from the program
                    }
                }

                // below is the if statement which will be checking if the player hits a zombie

                if (x is PictureBox && x.Tag == "zombie")
                {

                    // below is the if statament thats checking the bounds of the player and the zombie

                    if (((PictureBox)x).Bounds.IntersectsWith(player.Bounds))
                    {
                        playerHealth -= 1; // if the zombie hits the player then we decrease the health by 1
                    }

                    //move zombie towards the player picture box

                    if (((PictureBox)x).Left > player.Left)
                    {
                        ((PictureBox)x).Left -= zombieSpeed; // move zombie towards the left of the player
                        ((PictureBox)x).Image = Properties.Resources.zleft; // change the zombie image to the left
                    }

                    if (((PictureBox)x).Top > player.Top)
                    {
                        ((PictureBox)x).Top -= zombieSpeed; // move zombie upwards towards the players top
                        ((PictureBox)x).Image = Properties.Resources.zup; // change the zombie picture to the top pointing image
                    }
                    if (((PictureBox)x).Left < player.Left)
                    {
                        ((PictureBox)x).Left += zombieSpeed; // move zombie towards the right of the player
                        ((PictureBox)x).Image = Properties.Resources.zright; // change the image to the right image
                    }
                    if (((PictureBox)x).Top < player.Top)
                    {
                        ((PictureBox)x).Top += zombieSpeed; // move the zombie towards the bottom of the player
                        ((PictureBox)x).Image = Properties.Resources.zdown; // change the image to the down zombie
                    }
                }

                // below is the second for loop, this is nexted inside the first one
                // the bullet and zombie needs to be different than each other
                // then we can use that to determine if the hit each other

                foreach (Control j in this.Controls)
                {
                    // below is the selection thats identifying the bullet and zombie

                    if ((j is PictureBox && j.Tag == "bullet") && (x is PictureBox && x.Tag == "zombie"))
                    {
                        // below is the if statement thats checking if bullet hits the zombie
                        if (x.Bounds.IntersectsWith(j.Bounds))
                        {
                            score++; // increase the kill score by 1 
                            this.Controls.Remove(j); // this will remove the bullet from the screen
                            j.Dispose(); // this will dispose the bullet all together from the program
                            this.Controls.Remove(x); // this will remove the zombie from the screen
                            x.Dispose(); // this will dispose the zombie from the program
                            makeZombies(); // this function will invoke the make zombies function to add another zombie to the game
                        }
                    }
                }
            }
        }

        private void DropAmmo()
        {
            // this function will make a ammo image for this game

            PictureBox ammo = new PictureBox(); // create a new instance of the picture box
            ammo.Image = Properties.Resources.ammo_Image; // assignment the ammo image to the picture box
            ammo.SizeMode = PictureBoxSizeMode.AutoSize; // set the size to auto size
            ammo.Left = rnd.Next(10, 890); // set the location to a random left
            ammo.Top = rnd.Next(50, 600); // set the location to a random top
            ammo.Tag = "ammo"; // set the tag to ammo
            this.Controls.Add(ammo); // add the ammo picture box to the screen
            ammo.BringToFront(); // bring it to front
            player.BringToFront(); // bring the player to front
        }

        private void shoot(string direct)
        {
            // this is the function thats makes the new bullets in this game

            bullet shoot = new bullet(); // create a new instance of the bullet class
            shoot.direction = direct; // assignment the direction to the bullet
            shoot.bulletLeft = player.Left + (player.Width / 2); // place the bullet to left half of the player
            shoot.bulletTop = player.Top + (player.Height / 2); // place the bullet on top half of the player
            shoot.mkBullet(this); // run the function mkBullet from the bullet class. 
        }

        private void makeZombies()
        {
            // when this function is called it will make zombies in the game

            PictureBox zombie = new PictureBox(); // create a new picture box called zombie
            zombie.Tag = "zombie"; // add a tag to it called zombie
            zombie.Image = Properties.Resources.zdown; // the default picture for the zombie is zdown
            zombie.Left = rnd.Next(0, 900); // generate a number between 0 and 900 and assignment that to the new zombies left 
            zombie.Top = rnd.Next(0, 800); // generate a number between 0 and 800 and assignment that to the new zombies top
            zombie.SizeMode = PictureBoxSizeMode.AutoSize; // set auto size for the new picture box
            this.Controls.Add(zombie); // add the picture box to the screen
            player.BringToFront(); // bring the player to the front
        }

    }
}

bullet.cs – bullet class full code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using System.Drawing;
using System.Windows.Forms;

namespace zombieShooter
{
    class bullet
    {

        // start the variable

        public string direction; // creating a public string called direction
        public int speed = 20; // creating a integer called speed and assigning a value of 20
        PictureBox Bullet = new PictureBox(); // create a picture box 
        Timer tm = new Timer(); // create a new timer called tm. 

        public int bulletLeft; // create a new public integer
        public int bulletTop; // create a new public integer

        // end of the variables

        public void mkBullet(Form form)
        {
            // this function will add the bullet to the game play
            // it is required to be called from the main class

            Bullet.BackColor = System.Drawing.Color.White; // set the colour white for the bullet
            Bullet.Size = new Size(5, 5); // set the size to the bullet to 5 pixel by 5 pixel
            Bullet.Tag = "bullet"; // set the tag to bullet
            Bullet.Left = bulletLeft; // set bullet left 
            Bullet.Top = bulletTop; // set bullet right
            Bullet.BringToFront(); // bring the bullet to front of other objects
            form.Controls.Add(Bullet); // add the bullet to the screen

            tm.Interval = speed; // set the timer interval to speed
            tm.Tick += new EventHandler(tm_Tick); // assignment the timer with an event
            tm.Start(); // start the timer

        }

        public void tm_Tick(object sender, EventArgs e)
        {
            // if direction equals to left
            if (direction == "left")
            {
                Bullet.Left -= speed; // move bullet towards the left of the screen
            }
            // if direction equals right
            if (direction == "right") 
            {
                Bullet.Left += speed; // move bullet towards the right of the screen
            }
            // if direction is up
            if (direction == "up")
            {
                Bullet.Top -= speed; // move the bullet towards top of the screen
            }
            // if direction is down
            if (direction == "down")
            {
                Bullet.Top += speed; // move the bullet bottom of the screen
            }

            // if the bullet is less the 16 pixel to the left OR
            // if the bullet is more than 860 pixels to the right OR
            // if the bullet is 10 pixels from the top OR
            // if the bullet is 616 pixels to the bottom OR
            // IF ANY ONE OF THE CONDITIONS ARE MET THEN THE FOLLOWING CODE WILL BE EXECUTED

            if (Bullet.Left < 16 || Bullet.Left > 860 || Bullet.Top < 10 || Bullet.Top > 616)
            {
                tm.Stop(); // stop the timer
                tm.Dispose(); // dispose the timer event and component from the program
                Bullet.Dispose(); // dispose the bullet
                tm = null; // nullify the timer object
                Bullet = null; // nullify the bullet object
            }
        }
    }
}