With the #5 pick I am taking:
Pol Pot. He successfully genocided roughly 2 million Cambodians over 5 years which was 25% of their population. He was especially brutal to children, killing them by beating them against trees or tearing them limb from limb. He couldn’t have them growing up and avenging what he had done to their parents after all. He then piled the bodies in the open to scare people into submission. Add in the fact that he had a privileged upbringing and education and this guy was a real asshole.
@Riverman your turn.