Being unable to move diagonally feels really odd, especially when player movement is so slow in the first place. I also didn't feel like I was "reviving" the zombies because they just turn into invincible black statues. Credit where it's due: you added a form of difficulty ramping, which puts this ahead of a number of the endless/highscore-based games on here. It does feel like hitting a wave where you simply lack the ammo to make it through is a foregone conclusion, though, since the number of zombies you need to "kill" increases every wave and you can't seem to replenish ammo from crates or anything.

I don't see the theme, you say "you kill the zombies by reviving them" but I didn't get that when playing. The player moved very slowly for me and it felt like the primary gun was a bit useless when you have the secondary gun be so powerful.