I think you need born psychopaths who are abused badly themselves to produce most of these freaks. "Worst" doesn't really have much context once you're talking about people who have raped, tortured and killed at random numerous times, probably just a matter of how much they got away with.
There are a few in South America who preyed on children and have a disturbing amount of victims, kind of hard to get worse than that. I think Ted Bundy and the Green river killer are up there for North American cases.
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