Wow, this game is A LOT of fun! It reminds me a lot of 'Nuclear Throne'
Also, what's freaky, is that the number of zombies that come after you after about 2200 makes the screen look like a "Magic Eye"
It's not that's game breaking but it does add to the tripiness in a good way.
I think this could be developed into something bigger, with more guns, power ups, different zombies (with other creatures of the night that act differently.), and different friends with different abilities.
Fantastic work, either which way. Thank you for sharing your game with us all!