Well, if we're talking about an all out villain and not a tragic on (see Nightmare Moon) I'd say a sense of irredeemability is a defining trait that shows them in a contemptible light no matter what they do.
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Take Crocodile's teamup with Luffy. It's not a sense of alliance but rather that Crocodile is still a monstrous human being that could easily turn on Luffy in a second, but he's so badass while he does it that you're both sickened by his actions and impressed by his skills.
The other aspect I like is they're not just a villain, they're motivated into action. For as much as the hero struggles to prevail, the villain works to succeed in the own plan. I especially enjoy when a villain's goal drives them so much that the hero starts as just a nuisance that builds and builds and pushes the villain to more and more drastic action that hopefully leads to an epic climax (ex. Chrysalis)
Given that, I try very hard to avoid making my villains the antagonist just because they're evil. It's why I Sombra so much because his villainy comes off more as a force of nature or a wild animal than a menacing villain. Yes, in the past he's shown being brutal and his failsafes in the Crystal Kingdom are intriguing, but they're diminished by his appearance as a literal shadow that looms over the Kingdom. Yeah, that's a little scary, but not in the same direct way Chrysalis, Discord, and Nightmare Moon were presented.