I like the concept! I love the little visual of the many slowly turning more undead as he takes damage. That is a really nice touch.
I have two pieces of feedback. One, the zombies are VERY hard to kill XD I think it would feel very satisfying if the flame thrower just melted through them. So it made it very difficult to progress because I would run out of ammo for the flamer and killing them with the gun is just a little bit too slow. Two, I think having WASD movement directions line up with the camera instead of the world would make moving around a lot more intuitive. I kept thinking I was doing something wrong when I pressed W and my character would move left.
Great job! :D