Yes, contrast and humanity.
Absolute evil is boring (and unrealistic). He's bad, we're good, gg.
You want your antagonist to be opposed to your protagonists because they have conflicting goals that are more complex than, antagonist hates life/likes killing and the protagonists like living. Sure, you can have your Sauron but it's Gollum
that's the interesting antagonist character in LOTR.
He/She should be conflicted. They know that they're doing things that, in as close to a morally objective sense as can ever be established, are considered 'bad' BUT:
A) They don't care because of a warped worldview (a la the Comedian)
B) They feel that they have to. Greater good. Revenge. Slave to the Precious That sort of thing. (a la Ozymandias)
I mean, really this question answers itself when you ask, who is the more interesting character? Xykon or Redcloak? Who makes you think more and engages your brain in the story? The Jokeresque psychopath who kills for fun and obviously must be stopped? Or the guy who just wanted a fair shake for his downtrodden people, did a few bad things to try and get there, realized that if he ultimately failed then his early misdeeds would be unjustified and so resigned himself to playing out his hand to the end regardless of what was required to succeed and cognizant of the likelihood of his own damnation?
I'll let YOU decide. (but the answer is RC)
Anyway, just my opinion on the topic. Hope it helps and good luck!
Pro tip: Check out Watchmen for great examples of heroic antagonists - most of the characters work as well as villains as they do heros (hence the title