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!
*WASD* – move
*down*, *up*, *left*, *right* or *arrows* – move
*mouse* – look around
*space* – jump
*shift* – run
*0*..*9* – change guns