Zhang Xianzhong takes the top when it comes to insanely and irrationally violent psychopaths. He lead a peasant revolt in Shaanxi and was able to conquer Sichuan in 1644, where he acted kindly at first and was well-liked before going postal on the entire fucking area, killing approximately 75% of the population. Zhang ended up getting killed by the Qing dynasty after attempt to return to his hometown in protection against the Qings led to a confrontation with him getting killed after being betrayed by a Sichuan-born officer on his side. If we butterfly away that confrontation, how would he have faired against the early Qing dynasty? They were only three years into their reign and Zhang was quite the commander, and his reign of terror stroke fear into the hearts of men.
I appreciate any input.