How to Unlock The Cannibal
To unlock Cannibal, you need to kill 20 people on one level. You also need to exit the level.
The easiest way to get Cannibal is to put on using the “Everybody Hates You” and “Low Health for All” mutators, and equip Soldier or Gorilla (or any other strong character). Soldier for Ranged, and Gorilla for Melee. Since everyone has significantly reduced health, the Soldier’s Machinegun will take everyone out pretty easily, and the Gorilla can kill anyone with one punch. Another perk of using Soldier is if you go under 20HP, you will regenerate, so it’s nearly impossible to die.