Roman Equivalent of Stalin or Hitler?

I don't want to be insensitive to the horrors committed by Stalin or Hitler, but I was wondering who in Roman history would be considered the worst of the worst much like these men?

edit: We view Stalin and Hitler as horrible based on how many people suffered or died due to their actions and policies. Also, how inhumanly they treated others. So is there a Roman analog using this standard?

edit 2: woah. This post took off. And there's a surprising variety of answers on this topic. Thank you to all who have responded. If anyone knows of any good literature on this sort of comparison I'd be interested to know.