I really like the game. I find it very enjoyable, but I does need more things and some fixes which I really recommend;
-Add more guns. The game need more guns if would be more fun.
-Add an editor or something, creating your own map
-You should add a small animation like blood dripping from the sliced area
-I don't know why the head always pops outs even if I just slice their torso. I would fix that.
-Add more types of enemies, an armored one (need to slice a specific area), a very fast zombie, etc etc.., keeping the player more focused not just plain jumping and shooting.
-Lower just a little the zombies' difficulty.
If these things are done, then you sir have a solid 5/5 game.