Why is evil attractive?

When I was a kid I only shipped myself with villains (Iggy Koopa and Fawful from Super Mario to expose how embarrassing I was). Just like you're saying they're just much more interesting as people, they have a very human moral conflict that heroes lack. Especially when it comes to Super Mario since it's my main fandom, Mario is just a husk. He's been made for the player to project on, but he still doesn't have much to bring to the table.

Bowser however makes you question a lot of things. Why does he kidnap Peach, is he lonely? And he has a son we wonder where he came from that he loves even though he's "the bad guy". Sorry I went on a Mario rant for a second lmfao- Point is that villains appear to be as morally gray as all of us. (That's why we worship loser characters like Luigi too, but that's another side of it).

So far I've seen a few theories I can get on board with when it comes to this and I think I generally fall somewhere with 1 or 2:

1. We don't know them. They're mysterious. We don't get as much screen time with them and we don't get enough back story from them. We're left wanting more. It's often noticeable that when you do suddenly get backstory from the villain, particularly in shonen anime fandoms, their popularity drops. This is a common theory for why villains are often more popular than heroes. We don't know them so we can project onto them and make up our own ideas for them. Linked to the blank main character.
2. Catharsis factor and power fantasies. Sorta similar really. Sometimes it's just fun to be bad. Much as we love seeing the bad guys get when they deserve, we also enjoy watching characters suffer. Even in your mind or through another, lashing out when you've had a bad day, getting rid of people who've been rude or who bullied you at school or at work ect. If you've not even thought about it in passing I'm not sure you're human. And you can let this out through the villains. And as a writer "I'm upset so I want my characters to hurt so I need a great villain" is also a thing. It fun and relieving to be able to write a character who can just get away with anything, at least for a bit.
3. The "I can fix him". Probably most common with teenage girls.

queer coding!

i mean, not entirely... but a lot of people have come to relate to and celebrate villains such as disney hades and scar due to their gay-coded traits making them... a lot of fun, and a complicated point of recognition for a lot of lgbt people. the flourishes and snark of a villainous old queen (in the gay sense) are certainly sexy

theres also the idea that (straight) women are drawn to villains bc they like a 'project' - a damaged character they can work to 'fix.' i dunno how much weight that holds, but it may be true for some people - theres certainly a lot of 'i wanna wrap this damaged baby up in a blanket'

theres also power fantasy tied up in villains - theyre badass, through them you can vicariously throw off taboo and embrace a violent nature. their enticement might also tie into kristevas theory of abjection; the abject is (roughly put) the other, the Not Us, and the stories we tell work to separate the object from the abject through reinforcing taboo and morals. however, in the separation there must first be a blurring of the boundaries, and the abject is simultaneously repulsive and alluring............... im still gathering my Thoughts on this, i just read about it today

for me personally, its a lot of that last suggestion - theyre a great outlet for violent impulses. i connect with their common origins in betrayal and alienation, and theres recognition in their bitterness. a lot of villains we see in media also end up manifesting marginalisation - a la queercoding - and because of the various ways ive been marginalised in life, they naturally speak to me.

that said, i feel far more for the old generation of villains like scar or magneto who were outcasts more than anything over some more contemporary villains like kylo ren. there seems to have been some kind of post-twilight shift to the fetishisation of unstable-to-abusive men in our villain loving. i just want traumatised bisexuals in tights!

Plays the Sound of Scientific Triumph, BWAHAHA!!!

I like this quote by Simone Weil in regards to the attractiveness of evil.

“Imaginary evil is romantic and varied; real evil is gloomy, monotonous, barren, boring. Imaginary good is boring; real good is always new, marvelous, intoxicating.”

villains have more fun. They're stylish and depending on what type of villain they are they sometimes angst less than the hero. They're more intelligent typically or at least have cool plans. sleek constumes, good backstories so you can feel bad for them. they're also generally more motivated. The hero usually just wants to stop the villain, but the villain wants to make their dreams come true.

the villains sometimes have more systemic power which can be sexy. also teenage girls sometimes just want to do whatever they way which makes them similar to evil villains.

I have a LOTTTT of opinions about this an its too late and I'm not articulate enough to give the essay that this subject deserves (cause truly we could talk about this for daaaaaayyyyyssssss) so imma just go with my favorite answer

BDSM

Villains are... very seductive if I do say so myself :'D They're usually there to influence/test your good morals? Plus their confidence really boosts their aura (okay I'm getting into it too much)

Sometimes I'd like to see a story involving a "villain" as the main character for once. They being bad, not because of whatever negative backstory, but because they are just really like that and want to do what they do, and then succeed in their goal.

Probably cause they are everything the protagonist isn't. A protagonist is suppose to be the one whose got their shit together while villains don't, villains are usually bizarrely eccentric and over the top when protagonists are humble and down to earth so they stick out like a sore thumb. Villains make it about themselves, me, me, ME! so you gotta stare at them all the time and it becomes endearing seeing this trainwreck run around and destroy stuff all while flaunting themselves.

Or Hybristophilia for some people... see a doctor about that...

I think some of it comes down to the fact that a lot of villains become such because of them being wronged, or they feel like they've gotten the short end of the stick. Many times I think we feel that way - like life has dealt an unfair hand, and sometimes it feels good to get revenge. So we relate more to the villain. Watching some of the DC TV shows, I've had a hard time with some of heroes because they're so good at being the bigger person when it comes to giving the bad guy what he deserves. I think that's what makes heroes hard to relate to sometimes. True, we all want to be good, but in many ways it is easier to be bad, or teach someone a lesson. In many ways I get jealous of the good guys who can come up top over their faults - not because that makes them unrealistic, but because I want to be that way, and feel like a failure because I haven't reached it. I don't normally like siding with villains, but I do see so many of their qualities that are normal for anyone to feel, whether that be betrayed, hurt, lonely, incompetent, looked down on, discredited. They're all feelings that are worth overcoming. Villains just do it the wrong way, which again, outlashing and hurting others to justify your hurt, and not all villains do this, is something that's easiest to do.

Evil characters usually hold an undercurrent of self confidence that in otherwise morally upright characters people love to read as conceited and as a flaw, but in villains, it's hella appealing. Why the double standard? I have no idea. Maybe we're so obsessed with "good" people being morally perfect and milquetoast that any slight indication of a defect makes people turn away from them.

I make it a purpose to have well-written villains in my own stories. I feel like the reason we as readers enjoy the bad guys so much is the attraction to the forbidden. Villains have far more freedom than heroes and in most ways, are far more important than the heroes are. Villains act, heroes react.

Typically it's nice when you can relate to a villain. Maybe they just had a bad day, or a cause they're fighting for that "they" believe is just. After all, every villain is the hero of their own story.

I know what you mean, for me villains are just more relatable, they're a person that may have followed the wrong path, tragic hero, their backstory is what pulls people in as to how they became the person that is in the "wrong" in their story.
people in real life have different points of view and they at times believes that they are doing the right thing but the end up the having questionable motivations. i feel that villains are wonderfully complex like real humans, they have layers that make them so interesting.

Villains are free in their actions and can do everything they want, because they are not bounded with moral, as heroes are. That's why they can be more interesting.