Its been discussed that Hitler and the Nazis were about the worse thing that ever came along but pre around 1940, who was the terrible person in history? The closest I can see is maybe Napoleon?
You're in luck – this question has come up here before on a couple of occasions. So, while there is always more to say, you might like to review the most extensive of these previous threads, led by u/Georgy_K_Zhukov, while you wait for fresh responses to your query.