I don’t know 100% why this is. Maybe it’s just because I find that they’re usually more interestingly written, as long as they’re not 2-D Saturday-Morning Cartoon villains (which have their place, but they’re not my favourites). Maybe it’s also because I just find them more relatable! Villains pretty much always lose, they’re set up to fail. But it’s also because they’re not infallible. Sure, the hero may get tricked sometimes, he might fall on hard times, even hit rock bottom. But he always gets up, even if he becomes a martyr for his cause. There are always the heroes that are written as morally ambiguous or straight up just ‘chaotic good’ (i.e., “I will fight for what I believe in, even if I do it in terrible ways” - see pretty much every good guy in Frank Miller’s Sin City), and those are fun too. Some of my favourite antagonists are the ladies from Asian horror (mainly Korea and Japan) - Jin-Hie from Pon, Kayako from Ju-On, Sadako from Ring (the novel, she was way more interesting in that). Maybe because these ladies cause some of the most outrageously evil things to happen, but even Sadako has relatively humble beginnings. It’s just that they’re so oppressed by their world that even in death they continue to hurt those that hurt them. That’s sort of empowering. Especially with stories like Ring's and Ju-On's, where there are temporary or small victories but the good guys never really 'win' (this is a theme you see in stories like Kairo, too, which didn’t even have a ‘true’ antagonist).
Tumblr loves to fawn over villains. Loki from The Avengers is a pretty recent example, but you’ve also got fans of characters like Voldemort, the Malfoys, and Snape from the Harry Potter series, and, to a lesser extent, characters like Ganondorf from The Legend of Zelda. Whether it’s because they’re framed as genuinely sympathetic characters, or because our generation just seems to really like to fetishize evil, I know I’m not alone in preferring villains to heroes.
But I think many of us have very different reasons for it.