Until only recently, with the introduction of Erik Killmonger and the elevation of Thanos, the Marvel Cinematic Universe was routinely criticized for its lack of compelling antagonists, leaving only Loki as a shining star of villainy for much of the franchise's first decade. Played by Tom Hiddleston, the treacherous adoptive brother became an immediate fan favorite with his 2011 debut. But over the course of five films, Loki experienced arguably the most intriguing character arc in the MCU, from a impetuous young trickster weary of living in his brother's shadow to megalomaniac would be-conqueror to unlikely ally. His sacrifice in Avengers: Infinity War, while devastating to audiences, completed his journey, to hero.
So why, with a single sentence, has Marvel undermined that arc, attributing the god of mischief's diabolical actions in 2012's The Avengers to, well, mind control?
In Loki's official MCU biography, the studio essentially confirms a long-running, if not necessarily prominent, fan theory, that the scepter he wielded, which contained the Mind Stone, didn't merely allow the trickster to control the minds of others, it also influenced the Asgardian, "fueling his hatred over his brother Thor and the inhabitants of Earth." That phrase may be viewed as vindication by some fan theorists as vindication, but it robs Loki of his agency in The Avengers, and diminishes his ultimate redemption in Thor: Ragnarok and Infinity War.
My Brother, My Enemy
Introduced in 2011's Thor, Loki was immediately defined by his casual malice. He was spiteful and scheming, sure, but it's only after Thor is banished to Earth and Loki discovers he's not actually Asgardian that his machinations become sinister. However, all of his terrible actions were merely Loki's attempts to prove his worthiness to his adoptive father, Odin. He doesn't end the film cackling or vowing revenge, but instead quietly sobbing and trying to explain himself.
When Loki returned the following year in The Avengers, it wasn't purely as a villainous figure. Thor insisted the Avengers save Loki from himself, not kill him. His casually destructive acts leave hundreds dead in New York City, but his plot to claim a throne for himself technically draws him out from beneath his brother's shadow -- in the worst way possible. Yet that's grounded in the same emotions, and resentments, as his earlier actions, and fuels even more bad decisions, until Thor finally escorts him back to Asgard.
No matter what he's doing or what side he says he's on, the audience know the kind of destructive potential Loki possesses, especially after The Avengers. It also paints in a more tragic light his eventual (partial) reconciliation with his brother, showing him as a character with the capacity for change, even as he resists such growth. He's still motivated by ego and driven by a quest for power, but there's an internal understanding of his own faults by the end of Ragnarok, and an attempt to, if not right, then at least not repeat them.
Blame the Mind Stone
What made Loki such a compelling figure was his growth after the events of The Avengers. He could have easily been confined to prison and forgotten as the MCU marched on without him, but there remained layers to the characters for filmmakers to explore. For example, his love for his adoptive family bled through his villainous exterior, manifesting in his reaction to the loss of Frigga and his heartfelt farewell to his brother in Thor: The Dark World, and in his heartbroken expression to Odin's passing in Ragnarok.
By the end of Thor: Ragnarok, his efforts to rise above his worst impulses and do good strike a chord with audiences, because such growth is earned. And when he sacrifices himself in Infinity War, it matters. But by removing Loki's agency as a villain in The Avengers with a throwaway, "the Mind Stone did it!," Marvel has weakened that arc, making the some of the changes experienced by the character feel unearned.
Look at it this way: The Loki of Thor would have absolutely allowed Thanos to crush his brother's skull in the opening minutes of Infinity War. It took four movies of growth for the god of mischief to admit he genuinely loves Thor, and to arrive at a place where he would make the irrational, yet entirely understandable, decision to give up limitless power and possibly doom the universe, all in hope of saving his brother. Part of that journey for Loki has involved betraying and abandoning Thor, only to be welcomed back each time. It's what led him to a triumphant return in Ragnarok to rescue the people of Asgard, and helped him to acknowledge that no matter who his biological parents were, he is an Asgardian.
Of course, he could have been lying in Infinity War, but that's because he's a trickster god, and we can never really trust what he says. But Loki's nearly decade-long arc reveals a very different character than the one we met in 2011. His villainous actions in The Avengers, fueled by his own desires, were an important part of that journey, informing both his character and Thor's. If Loki's atrocities are suddenly waved away as the result of an outside influence, then we have to question for what, exactly, he needed to be redeemed.