Nice art but,
The art was great and the merc thing provides some reason for being there.
Usually I like endless hordes of zombies but this game get pretty boring after about 5-10 minutes. The reload mechanic makes the game too easy since you can instantly reload your weapon. I suggest a delay.
As other people have stated there should be stages/goals that you can achieve so you feel there is some point to this. Giving us something to accomplish makes the game more fun.
What is the point of shooting the zomibes in the head if they still come after you! Please make head shots worth something.