Thor is a movie about a dude taking the news he’s adopted very badly.
Okay, but no, seriously. I think the reason Loki resonated with basically fucking everyone - despite being such a little shit - is (in addition to Tom Hiddleston’s fucking face), is that he is obviously in so much goddamn pain throughout the entire movie. He’s fucking crying as he’s goading Thor into a fight.
Loki’s probably spent his… entire life feeling inferior to Thor because his talents lay in magic and illusions instead of beating the ever-living shit out of things with a hammer, feeling different for reasons he can’t entirely understand. So then he grows up, and finally discovers why, and it’s a reason he has no control over - it’s something he can’t change, he’ll always be a frost giant, and so he’ll never, ever be equal to Thor or even his father’s son, not really. And it’s kind of incredibly horrifying, too, because Loki was right to think Odin had to have a reason, because the fighting was over; he had no reason to steal a baby; the only reason Odin took Loki was because he thought he would be useful in the future. And Loki is small; in the comics Laufey had hidden him away because he was ashamed his son was a runt, so even among the frost giants Loki would have always been inferior.
And the way I see it, Loki didn’t expect the thing with the frost giants to go as far as it did, and so he ends up in a situation he wasn’t altogether prepared for (or even wanted), right after he finds out all of that. He’s in an incredibly fucked-up spot, mentally, so he kind of loses it and concocts an equally fucked-up plan as a last-ditch effort to prove that he does belong in Asgard, that he’s just as good as his brother. And by the time Thor shows up again, it’s all gone to hell, and Loki knows it, and he’s just making an increasingly desperate series of really poor decisions. And then he ends up hanging off the end of a staff over a wormhole, and suddenly he can do nothing but face the truth he’s been trying to deny since even before he discovered the frost giant thing: that he would never, ever have been as good as Thor in the eyes of his father, or anyone else in Asgard. And so he lets go, and after everyone is finished bawling their goddamn eyes out for the little shit, and he shows up in Avengers, he’s had a very unpleasant trip through a fucking wormhole of his own making, with plenty of time for that bitterness to turn to hatred, and if he would never be as good as Thor, why not just fucking embrace it?
And the worst part is even if Loki hadn’t turned out to be such a complex character, I still would have liked him more than Thor. He’s smart and talented and powerful: he was different, but not inferior, and that alone should have made him special. But because he grew up in a culture that valued strength over everything else, he was always second best, always inferior, doing “tricks” instead of fighting. And that is fucking awful.