I played through to the end, there's some pretty nice ideas but I felt that the main difficulty came from the controls. The ground movement is too slidey, and jumping is too floaty, which makes precision movement difficult.
I gave this 0/5 for all the Newgrounds stuff on it, but then I increased it to 4/5 because of the people walking behind the guy at the start. Also, the rest of it was entertaining too :D It's a shame you didn't turn this into a full game.
It would be better if the gun followed your mouse, instead of jumping to it when you click. also, upgrades and other weapon types are pretty much essential for this type of game. The graphics are nice, though.
This is a huge improvement over your earlier games in terms of design and gameplay. What I felt it was missing was a proper campaign mode, where you go from A to B and kill zombies on the way, rather than the short survival style levels. Also, displaying gun stats would be nice since most people don't know enough about guns to know which weapons are better to pick up.