Edit: Looking for instances like we are seeing now. Where maybe someone walked into a market or a public gathering and just started assaulting random people.
Thanks for a great question. Here are some previous answers to similar questions to get you started. I focused on a serial or spree killer angle instead of a deranged dictator angle but If you were looking to find out who was Stalin before Stalin I'll gladly help there too.
Were there any serial killers in Ancient Rome or during the Middle Ages?
Were any werewolf or other creature myths based on real events?
Has there ever been widespread cannibalism?
The answer is a definite yes.