sorry man this game is very boring
Im sorry but this game is justa clone of many others, there is no idea at all, im a fan of senseless violence games, but then there has to be some unique elements, this doesn't have one.
Suggestions: Add some platforms for the player and Zombies to jump onto.
Let the player set "traps" where they have use their brain as well as aiming good.
Make it a challenge to shoot the zombies, its far too easy, no challenge = no gameplay.
Hope the advice helps !