He plotted to have his father killed and lied to keep his brother banished to Earth. When it looked like he might escape, he sent a giant robot warrior to take him out.
Then, in The Avengers, he tried to wipe out humanity by forging a partnership with an evil alien race, opening up a wormhole to allow them to invade Earth with giant serpentine beasts.
When Tom Hiddleston calls Loki a "psychopath," he's not kidding.
"As a character, he's so grand in scale and dimension," says Hiddleston, who returns as Thor's adopted brother for a third outing in the upcoming sequel Thor: The Dark World, which hits theatres in New Zealand next week.
"Obviously on a human level he is a psychopath who actually has a very intricate psychology in terms of the make-up of his emotional damage.
"And yet, he is a god, and we are dealing with gods, monsters and ancient beings. In the Norse myths he's the incarnation of the trickster.
"For an actor, it's gold to dig around in this stuff."
Watch footage of TimeOut's interview with Tom Hiddleston:
Aside from the inspired all-in sprawl of The Avengers, Thor was one of the best films in the Marvel superhero universe, thanks to Chris Hemsworth's grizzly portrayal of the muscle-bound lunkhead, some jaw-dropping set pieces involving frozen armies and otherworldly beasts, occasional outbursts of genuine belly laughs with its fish-out-of-water love story, and a mythology that rivals Game of Thrones for its intricacy and intensity.
But what really set it off was Hiddleston, who played the wiry, snivelling Loki with a mischievous twinkle in his eye and a "who, me?" smirk on his face.
He's been bad in the past, but what Hiddleston gets up to as Loki in Dark World may even top his antics across Thor and The Avengers.
At a preview screening, TimeOut saw 30 minutes of footage including one gruesome scene involving Loki and his brother that was so shocking it sent surprised gasps through gathered media.
With the film yet to hit cinemas, TimeOut is sworn to secrecy, but it's fair to say Loki isn't planning on mending any of the rifts that have developed between him and his brother.
Hiddleston admits that, in this case of sibling rivalry taken to epic extremes, Loki might be the worst little brother ever.
"The complication is that they're not actually siblings - they were brought up as siblings until Loki found out he was adopted," says Hiddleston, referring to Loki's discovery he was born a descendent of Frost Giant leader Laufey before being found and raised by Thor's father Odin (Anthony Hopkins).
After the shenanigans of Thor and The Avengers, the 32-year-old says the only redemptive signs for Loki come from his interactions with his mother.
"You see a sliver of vulnerability in his relationship with his mother.
"At the beginning of this film, Loki is in prison, incarcerated, unseen, unheard and forgotten and nobody in Asgard wants to hear his name or visit him.
"Possibly the only person who still cares is his mother, and maybe he recognises that, and possibly that's where a twist takes place."
Hiddleston, who has theatre performances as Cassio (from Othello) and the lead in Coriolanus behind him, agrees there is an element of "Shakespearean grandeur" to Loki.
"A lot of Shakespeare's plays are about kings and queens and princes who are wrestling with each other for the same thing.
"Think about Macbeth, think about King Lear ... Shakespeare knew how to write good bad guys."
When it comes to what drives Loki's decisions, Hiddleston sums them up like this: "He feels hard done by, by people.
"He can't help himself in terms of wanting to betray people and give them as good as he got."
Hiddleston, who only had a series of minor TV parts to his name before Thor, says it didn't take long for him to agree to the role.
"It was a combination of factors: The chance to be in a big movie, the fact that Loki is an agent of chaos on a massive scale . . . it's nice to change my shape a bit.
"I look like a Scottish farmer most of the time ... to suddenly have black hair and to have a pale face and look different was an amazing opportunity.
"To work with [Thor director] Kenneth Branagh and Anthony Hopkins and Chris Hemsworth - who's become one of my best friends - it's been one of the most rewarding acting relationships I've ever had."
It's fair to say some of Loki's antics have become fan favourites - and Hiddleston says there are several moments that seem to stick in their minds.
"They love it when I turn up at the beginning [of The Avengers] and say, 'I am Loki of Asgard and I am burdened with glorious purpose', and people also like my incredibly abusive insult to the Black Widow (Scarlett Johannson) when I'm in prison, when I call her a 'mewling quim'.
"It is not a nice thing to call a lady."
And Hiddleston tells a nice story about watching The Avengers for the first time with a group of rescuers involved in the 9/11 terrorist attacks on New York.
"I was in New York when The Avengers came out, watching the film with a bunch of first responders in 9/11 and one of the great thrills was when they cheered for The Hulk when Loki gets Hulk-smashed - they loved that he was getting his just desserts.
"I love that my job as a bad guy in a film is to make the audience want me to lose. In that particular instance I realised I had done my job."
Who: Tom Hiddleston
What: Thor: The Dark World
Also starring: Chris Hemsworth, Natalie Portman, Anthony Hopkins, Idris Elba
When: Opens in cinemas October 31