If want to play as zombie and fight agains and interstelar alien then try Martian Zombie Survival in the great fps sci fi with option to play in 2 mods and fun.
Enjoy!
Instructions:
*WASD* - move
*down*, *up*, *left*, *right* or *arrows* - move
*mouse* - look around
*space* - jump
*shift* - run
*0*..*9* - change guns