This is very nice! It's like a very simple zombie shooter. The zombies can get very overwhelming very fast if you let them keep spawning, and I think that's something you might have been aiming for with this game!
I think something nice would be if the zombies reacted a bit quicker to the players' clicking. Whenever I hit them, I can't really tell if I hit them or not because it takes a few seconds for them to drop dead. I think it would be a lot more satisfying if they just did their drop animation right when I hit them.
I also think a "Restart" button would be very helpful!