Loki from the Thor movies, Sebastian Morgenstern from The Mortal Instruments, Severus Snape from Harry Potter, Kylo Ren from Star Wars, Killmonger from Black Panther—let’s admit it, we all love the bad boy. He’s always the hottest male character, the most intriguing, the one we all fantasize about. His are the bits we go back to read or watch, over and over. He’s the one we always sort of hope the heroine will end up with—or, at least, make out with—even though we know we shouldn’t. He’s the one we sort of hope will win; and he’s the one we mourn when, as is inevitable, he fails at the finish.
Why is that? The bad boy antihero is, objectively, an absolute jerk. He’s hateful and harmful and does things like skewer his dad with a lightsaber and toss his cousin off a waterfall. He’s the villain, however you cut it. When we meet him in real life, he’s an asshole, and, if we have any sense, we run screaming. So what makes the antiheroes of science fiction and fantasy—these hateful, murderous jackasses—so appealing?
Let’s start with the obvious: sex. The antihero is always alluring: the seducer. The fact that he’s evil only makes it better: he’s the forbidden seducer, and what’s forbidden is always irresistible. The antihero’s sexuality is often a lot more overt than the regular hero’s, and couched in terms of sizzling seduction: where the hero is saying, Let’s live happily ever after, the antihero is saying, Let’s retire to this shadowy corner here and do terrible things to each other. And who doesn’t occasionally fantasize about doing just that? The antihero allows us to exercise our fantasies, in a way that most heroes, constrained by the necessity of goodness and decency (at least to some degree) do not.
Similarly, the antihero is often more emotionally interesting than the hero: while the hero needs some basic heroic strength, the antihero can be weak. He’s screwed up, hiding dark secrets, deals with conspiracies and inimical plans. He’s often in a lot of psychological pain, and there’s usually a good reason for that. He’s dark, and it’s in darkness that mysteries gather. These mysteries might be horrible, evil, world-shattering; but they are mysteries, and thus utterly captivating. This is probably part of the reason why Kylo Ren has generated approximately a billion times more interest and speculation online than Poe Dameron: we know Poe the moment we see him. His actions aren’t always the wisest, but they are motivated by clear, noble intentions, and performed in a spirit of unselfish bravery. We know already how his story is going to end: he’s either going to lead the Resistance to triumph or die heroically in the attempt. So far, so predictable.
Kylo, on the other hand, is anything but predictable. He’s got secrets; secrets that are driving the story forward, 
secrets we are desperate to know and understand. In contrast to Poe’s wholehearted, straightforward devotion to the Resistance and saving the galaxy, Kylo is full of self-doubt, horribly split between his own desires. He’s both terrible and pitiful; laughable, even, in his pathetic aping of Darth Vader—a childish pantomime that he renounces. He commits unspeakable crimes, like killing his own father—and then expresses true, heartfelt sympathy and understanding for Rey’s plight. He saves her life by killing his mentor, Supreme Leader Snoke—and then launches a vicious attack on the Resistance when she refuses his honest pleas to join the Dark Side. At the end of The Last Jedi, he is Rey’s mortal enemy, vowing to destroy her—and he loves her still. Kylo Ren is many things, but he is not simple, he is not easily understood, and so he is a truly fascinating character.
All true antiheroes—as opposed to plain, straight up villains—are fascinating in this way. They make us think. Like Rey, we sympathize with their pain, even as we recoil from their evil. They ask us the uncomfortable questions: what if I were in this situation? Would I do any better? Unlike Voldemort, for instance, or Emperor Palpatine, whose evil is straightforwardly simple and inhuman, it’s only too easy to imagine oneself in the antihero’s shoes. We all have darkness in us, we all have rage—and we all have a breaking point, beyond which we cannot be pushed. There is always the possibility that we are, or will be, the antihero, rather than the hero, of our own story.
The antihero also embodies the hero’s darkness: the inner evil they must renounce, but can never entirely destroy. This is probably why the antihero is so often a close relation of the hero: Killmonger is T’Challa’s cousin, Kylo Ren is romantically involved with Rey, Sebastian Morgenstern and Loki are the brothers of their respective heroes. These close relationships emphasize the antihero’s connection with the hero; and they make it impossible for the hero to dehumanize or completely hate them. This is important, for the antihero is not a demonic villain with inhuman motivations à la Supreme Leader Snoke. The antihero is who the hero could have been if circumstances had been different. It’s only too easy to imagine T’Challa becoming as embittered as Killmonger if he were abandoned as a child in a world that treated him as a second-class citizen; Clary Fray might have turned out as psychotic as Sebastian Morgenstern if she’d been the one poisoned with demon blood and raised by a zealot. No antihero can be entirely condemned, either by us or by the hero; no hero can entirely sever their connection with their dark mirror.
Ultimately, this extends to us, the viewer or the reader, as well. We all have darkness; we all have weakness. What makes a hero rather than the antihero is how one chooses to act on that inner evil. We all hope to be able to resist that evil—we would all rather be Rey than Kylo Ren, T’Challa rather than Killmonger—but there’s always the possibility that we won’t. The antihero of fantasy and science fiction acknowledges that possibility, addresses our human evil, while warning of the terrible danger of succumbing to that evil. No one can say that any of our antiheroes lead good or happy lives; and that they always fail in the end underlines the ultimate message of fantasy and science fiction: while we cannot destroy our own darkness, it is never worth it to give in.