Loki is perhaps best known as the bad so big it brought The Avengers together. That's not just the case in some Disney movie. It's the basis for the very first Avengers tale -- spun when one-time Hulk sidekick Rick Jones brings Ant-Man, The Wasp, and Iron Man into a Loki-created conflict between Thor, and Bruce Banner's verdant half.
Since then, Loki has been the master of manipulation in the Marvel Universe. They've taken over Asgard who knows how many times, and turned heroes against each other to cover their tracks. During the Dark Reign storyline, Loki maneuvered the pseudo-stable Norma Osborn into becoming the Green Goblin again. Thus beginning the Siege storyline that capped off years of Marvel Comics events.
From here on out, things get... conceptual for the Trickster God. Siege actually ended with Loki repenting for their evil ways, and helping to save the day from The Void -- the dark side of the world's most powerful superhero, The Sentry. The super-villainous doppleganger tears Loki to shreds, thereby leaving very little doubt that they're dead. So, of course, Loki came back to life almost immediately. This time, however, his "aspect" inhabited the body of a young, and by all accounts innocent boy.
This Loki was actually a completely different person, while at the same time not a completely different person. It's all about how Asgardian gods are just stories, and change when the stories change... Look: all you need to know is that Kid Loki remembered nothing from his past life, besides a general sense of guilt over being such a dick.
That newfound conscience, and the drive to prove himself to the people who still didn't trust his fresh face, made Kid Loki a pretty swell guy! He was still a mischievous, two-timing, underhanded trickster, but he used those skills for the betterment of not just Asgard, but humanity as well.
That is... until you find out that the old Loki had planned all of this from the beginning. Kid Loki's heroic actions would make him trustworthy again, the original would "kill" his reincarnated self by subsuming his personality, and evil would win again.
The plan worked perfectly. O.G. Loki went back in the books, and nobody was the wiser. Of course that didn't erase all the good his successor had done. But that good did change Loki. This newer, fresher, Tom Hiddleston-er Loki was still influenced by perception -- by stories -- about himself. So by having to keep up the farce that he was still the "good" Loki, he slowly and inadvertently changed himself for the better.
While he still hasn't quite live up to Kid Loki's heroism, this latest version of the character is... Let's say more "morally grey" than previous incarnations. Or he will be, so long as he doesn't commit cosmic murder-suicide again.
Marvel movies have been living in the shadow of Thanos since the very first Avengers film. Remember that post-credits scene, with the purple pounder's shit-eating grin capping off two hours of Hulk catching Iron Man? If you don't, just know that we lost our collective goddamn minds.
The reason being that Thanos is about as close to a face for evil in the Marvel Universe as you can get. While most other villains have turned hero at one point or another, Thanos has remained a staunch Bad Guy. Honestly, we can't blame him. It's hard to step back from killing 50 percent of sentient life in the universe in your debut event. Which is exactly what Thanos did when he snagged the Infinity Gauntlet in the appropriately titled Marvel crossover, The Infinity Gauntlet.
Since then Thanos has gone on a galactic killing spree against his own children, culminating in the Infinity event by Jonathan Hickman. Wherein Thanos was responsible for not one, but two major Marvel cities, Wakanda and Attilan, getting sacked and destroyed, respectively. That didn't dissuade the United Nations from giving him free rein to murder entire, alternate Earths so that they wouldn't collide with number 616, which is... also something he did.
Through it all, Thanos worshipped death. Literally. He was in love with its physical personification, and did most of what he does for her sake. Death, for her part, didn't care much for Thanos's interpretation of her role in reality. Thanks to that, and the reality-ending events of Marvel's second Secret Wars, Thanos has since traded in death for total, straight-up nihilism.
Basically, it's hard to find anything redeeming about Thanos at all -- even for the big guy himself. Not one to back down from curiosity, however, Thanos once took his complete lack of moral fiber as a challenge. In the 2003-2004 series simply titled "Thanos," the Mad Titan spends a good 12 issues simply asking himself if he can be good. After all, since villainy hasn't worked out so well for him in the past, why not try heroism?
Of course, Thanos's definition of "heroism" involves lots, and lots of killing. Like, a whole bunch of it. He aids the Annihilation Wave -- a galaxy-devouring army of bugs -- in doing its dirty business, just to learn their ultimate goal. While Thanos does end up stopping the Annihilation Wave from destroying all life, he helps end trillions of lives in the process. After which he basically decides that heroism isn't really for him, but gains the satisfaction of knowing that he's a bad guy. To which we say: hey, at least he tried. That one time.