Thanos killed half of all life in the universe by snapping his fingers. No villain has ever murdered so many with so little effort or hesitation.

shuma bitches!!!!

Possibly Celestials, multi verse, and if Fox can be reaosned with Galactus and his harbingers.

Challenge will be managing aging actors.  We may see Iron Man or others die or retire to make way for younger one in the coming years.

In one book, which was retconned later (outside of comics in a blog), he did one better. Thanos isn't totally evil though, just has a mad obsession with death, it's been shown he has some compassion.

Anyway, Carnage might be sadistic and insane, Red Skull might be well... a Nazi but none of them compare to the true evil of the Marvel Universe.

Disney.

... no seriously, Mephisto is the personification of the devil in Marvel, he is evil. 

Going through your list:

Ultron - Knows no better. Bullseye, just a murderer (just sits on the wrong side of morals unlike Wolverine, a murder who sits on the other side). Apocalypse is actually heroic in some purposes, he loves humanity and wants it to succeed. Red Skull we know, Nazi. Barracuda? Really? Carnage is sadistic. Norman Osborne, deluded. Mr Sinsiter, too prideful. Zemo, Nazi. Dormammu, closest to evil out of the list.

Likely Carnage, since the guy has no regard for life, human or animal.