Hi there! Quite neat little gam you put together. It still needs some polish, but the basics are there. 1. Could you make the camera movement with the mouse restricted? It feels a bit bad on a 16:9 monitor that the player can't look as much ahead when looking north or south as east or west. The idea isn't bad but the numbers need a bit more tweaking. I also managed to look waaay ahead.
2. I don't know if the aiming isn't perfect or the bullets have a different amount of damage but thanks to this I sometimes emptied a full magazine of pistol bullets into a zombie and it didn't even get a scratch. This can feel frustrating. If the aim "isn't perfect" that should be indicated somehow with a crosshair or something. If the damage per bullet is different you could write it in the shop.
3. The shop could be called the arsenal or loadout so the player gets interested at first glance before hitting the play button.
4. The hitboxes are not the best defined. I don't know if the wall or the player has a large hitbox but I managed to get stuck on an invisible wall many times (mostly walking backwards.)
5. The description on the beginning should be skippable with mouse click or hitting a key on the keyboard. It takes up precious time from the player waiting through that every time they start the game.
This could be a fun game just needs some time and effort. :)