Marvel’s official character page for Loki has been modified. It isn’t exactly clear when the changes were made, but without any fanfare whatsoever, the theory that the Mind Stone corrupted Loki has been confirmed as canon. We’ll let fans read through the related passage, copied below from Marvel’s site:
Arriving at the Sanctuary through a wormhole caused by the Bifrost, Loki met the Other, ruler of the ancient race of extraterrestrials the Chitauri, and Thanos. Offering the God of Mischief dominion over his brother’s favorite realm Earth, Thanos requested the Tesseract in return. Gifted with a Scepter that acted as a mind control device, Loki would be able to influence others. Unbeknownst to him, the Scepter was also influencing him, fueling his hatred over his brother Thor and the inhabitants of Earth .
Hm. Yeah, okay. Why not. I don’t think you really need this explanation - he did some pretty bad stuff in the Thor movies, too, after all. But it does help explain the way he goes all-out villain in Avengers, and makes it easier to set up his series on Disney +.