2-bit?
Hmm. Very nice. Entertaining. I noticed some glitches, though. The first zombie I killed I shot about 10 times while he's lying there, even though you need only shoot him once and then it dies. Also, when you hit "d" quick you shoot, but when you hold it down for a bit, your sprite does a recoil reaction. I think you should tweak this, because the reaction is pretty neat. Try and change it so he recoils even with a short keystroke.