He did a bunch of really messed up stuff. Including, physically pushing a nation under the sea, forcing others to chose who he will slaughter, indiscriminately vaporizing millions of people with his vision, unleashing insane alien gods (from his prison) onto the earth...
Sabretooth should probably get a mention. He kills for the fun of it at times. He sliced up Harmony Young for nothing more than giggles and to piss Cage off. Silver Fox got the same treatment as did a bunch of others.
Except beating the FF properly, which was the only thing Doom needed to learn to begin with.
Anyhow, I'm going to mention Marcus, Kang's son, who mind-raped Ms. Marvel into loving him, then bred her with himself and was born from Carol's womb and then took her to limbo so they could continue loving each other in that mind-controlling incestual rapey way.
Did he rape his mom and mind-controlled her before she was born? Because Marcus is awesome in that jerk department -he also pulls this off without having anyone opposing him nearly at all-.
In terms of street-level baddies in Marvel, I think the worst of the worst are Bullseye, Crossbones, and the Purple Man. We're talking hundreds if not thousands of counts of theft, terrorism, rape and murder.
__________________ Land of the free, home of the brave...
Do you think we will ever be saved?
In this land of dreams find myself sober...
Wonder when will it'll all be over...
Living in a void when the void grows colder...
Wonder when it'll all be over?
Will you be laughing when it's over?
Hmmm... Does Dark Phoenix still hold the record for highest murder count in a single act of malice?
__________________ Land of the free, home of the brave...
Do you think we will ever be saved?
In this land of dreams find myself sober...
Wonder when will it'll all be over...
Living in a void when the void grows colder...
Wonder when it'll all be over?
Will you be laughing when it's over?
Thanos killing the entire population of his home planet of Titan and killing his mother has to be right up there... that alone with wiping out half the universe gives him top ten and prob top 5 for sure.. Top spot.. probably not, as he's not really EVIL per se... he just doesn't give a **** abotu anybody but death and himself...
Oh yeah the Thanos thing: what happened was that he wanted to know why he was born a devient so after years of hating his mother he decided to abduct her and operate on her womb to see what matter of uturus could make him like that, and for revenge. His father then stumbles upon her dissected corpse and soon after Thanos began his bombing campeign on titan.