Easy, but not
The game has tons of potential (see suggestions below)
The game is easy until the zombies start swarming. Even with the machine gun, it is very difficult to survive later in the game.
Suggestions:
1) create missions for the player.
2) when the player dies, they lose weapons and money (creates more of a challenge)
3) Instead of health replenishment over time, have the zombies drop a health pack.
3a) Bullets and guns can also be dropped by zombies.
4) more of an idea... maybe you can create a world after the movie "Land of the dead" where they know how to shoot weapons and use tools and such.
Good concept by the way. keep up the good work.