Serial killer Jeffrey Dahmer was done in by his uncontrollable lust for human flesh, the man who whacked him in prison 20 years ago told The Post, revealing for the first time why the cannibal had to die. Christopher Scarver — who fatally beat the serial killer and another inmate in 1994 — said he grew to despise Dahmer because he would fashion severed limbs out of prison food to taunt the other inmates.