Good use of the theme, I like that you're periodically switched between the lone survivor and the zombies. I like that some zombies also have weapons, makes it feel a bit more fair once you swap. I do wonder though, if you purposefully lose most of your health when you're the player, and then swap to zombies side, would that count as a win or a loss? Something to consider is players throwing on purpose ;)
Would be great if there was diagonal movement, although limiting the movement in the 4 cardinal directions did make it more difficult, albeit at the expense of the players comfort.
Well done! I'd love to see this game expanded on :D