We have a thread addressing the Hitler part here, with some lively discussion:
https://www.codohforum.com/viewtopic.php?t=255
As to the non-Hitler part, "evil" is obviously highly subjective, and almost none of the below is going to be satisfactory in any way, but its very "fun" to discuss. What is "most" evil? Does that even make sense? Which traffic light is the most "green" traffic light in the world? Which sunset was the most beautiful sunset ever? Topics like this can obviously go many different ways - what is the most evil thing to do - to a modern Western audience, its usually a toss up between murder, rape and pedophilia. Lets call this the Big 3. Using this metric, the "most" evil person is the person who meets all three of those criteria, the most number of times. End of story.
However...... There is obviously a scale-problem here, as its (probably) easier to commit mass murder at scale than, mass murder+rape+pedophilia. Therefore can you suggest that an X+1 time mass murderer out-evils an X-1 multiple time pedophile? I think alot of people wouldn't, for obvious reasons, but again some people might.
Breaking away from the "big 3", what about a mother who stuffs her new born baby in a trash can, or a freezer - a good case can be made for that to top those other three, because you have introduced a very specific type of betrayal of innocence.
There is also another factor in that i said "modern" Western audience. But to a modern "woke" Western audience, they would consider racism to be practically tied with the big 3, making it a big 4. Does slavery join this list to become the Big-5?
Another problem is that records are obviously incomplete - you just know there was some completely unknown tribal villager at some point who would outrank every known person in written records, but we don't know anything about him / her so they aren't in the conversation.
As for who is the most evil, Cain is a good option (if he existed - I'm not a theologian so you can tell me if he was allegorical or not, I just googled it and the jury seems out to lunch). If we are limiting this to only people we specifically know about from recorded history, and the angle you probably want to take must account for scale and proximity to the actual killings, then Genrick Yagodah, Yakov Sverdlov, Felix Dzerzhinsky & Lazar Kaganovich rank highly for their direct roles in the Red Terror / Gulags / Holodomor. Stalin is probably somewhere in the conversation, as is Pol Pot & Chairman Mao however those latter names obviously had a larger degree of "distance" between themselves and the actual killings, so again subjectivity is needed here.
Back to the final part there about Hitler again - Hitler obviously fails 2 of the big 3 crimes (or he fails 3 of the big 5), with racism being a given which i don't count as evil at all but you probably do so I'll include it, and for murder I assume you still believe in the Holocaust, i obviously don't so i don't factor that in here - however he did sign 6 execution orders, with a 7th being added by Goring (AH would have had the final say so lets call it 7). These execution orders were extrajudicial by nature, and as you said in the other thread, one of them was a former friend. Does that count as evil? Up to you, i guess, but to me no. He also signed Germany's Eugenics law into power (signed, but didn't draft it) - is that the same as murder? Who knows, but probably not.