Painkiller delivers exactly what you'd expect from a game like quake or doom but puts its own humor and twists into it that make it truly its own game.
-BLOOD and lots of it.
-Screw story, the story is superficial. You died, you kill monsters for God in inhumane ways. Welcome to Hell.
-Weapons that make your inner teenage boy squeal such as a stake gun that pins enemies to the wall, a shotgun that can freeze enemies and blast them into frozen chunks, or the painkiller staff which essentially is a blender.
-Big bad guys
-LOTS of bad guys
-Ragdoll physics (this is very fun when you are using the painkiller staff which makes them flop around before their bodies explode)
-Collectible cards that you get from completing different challenges in each level that when activated you get special powers or bonuses at the cost of some coins.
If you want story, intelligent AI, pacifism, or educational software then go elsewhere.
Otherwise enjoy you brainrotting mindless bloody murder game. :D