Art style is pretty sweet! Like the extinction effect of enemies.
How do you score points in the game? It looks a little randomized, makes no sense, hit a penny or just grazed.
It is not clear why it's necessary to destroy all zombies in exchange for life. It would make sense if the player was protecting something or anything like that. The level is also quite empty, the player has no need to move on it, as he can simply shoot from the center. Some new enemy type (such as zombies emerging from the ground) or new task could make the gameplay more interesting.