Wow - this was admittedly quite a dark game from the context. It was a bit too hard for me so from the time I spent playing this game, I'd like to start off with the positives:
1) Pretty good spritework for the characters as a top down game. Starting off on good ground - with more work and practice, you can turn this to a pretty good game.
2) The game reminds me of the blockhead zombie games with the gameplay. Simple concept and in terms of mechanics I think you nailed it pretty well. Aside from some obvious polish/balancing to limiting pistol rate of fire, number of enemy at one screen at any time - etc.
A noticable gripes I had with the game were:
1) Camera FOV - for this top-down shooter, you shouldn't make the camera be dragged by the player, instead the camera should face towards the cursor so when you aim up, move the camera upwards a bit. This will extend the player's line of sight better. You can also move the camera FOV to see more things.
But wow - quite a dark game. It actually plays quite well and I enjoy it but the execution of the theme is quite drastic if I do say so myself. Fun to play though - kudos!