The first Doctor Strange in the Multiverse of Madness trailer features an evil Doctor Strange, and there’s a good reason for it. One of the last shots of the trailer clearly shows the heroic Doctor Strange and the evil Doctor Strange in the same room, so the odds of the movie having the main hero become a supervillain are very low. The evil Strange is clearly an alternate version of the character, which ties into Marvel's ongoing Disney+ series.
The evil Doctor Strange shown in the trailer appears to be Strange Supreme from Marvel’s What If…? The fourth episode of the animated series showed an alternate version of Doctor Strange where Christine Palmer died instead of Strange injuring his hands. Desperate to bring her back, Strange begins to dabble in dark magic that involves absorbing countless mystical beings to make himself more powerful. The ritual turns him into a monster, both literally and metaphorically, and while he does bring Christine back, she’s horrified by the person he’s become, and Strange Supreme is left broken as his reality is destroyed around him as a consequence of what he was doing.
Based on all of that, the events of Marvel’s What If…? are most likely why there’s an evil Doctor Strange in Doctor Strange in the Multiverse of Madness. Strange Supreme is a man who sacrificed everything for someone he loved, only for it to all fall apart in the end. He lost everything, and with all that he did to get to that point, it’s too late for him to go back to his old life, so he has no choice but to stay locked in the life he etched out for himself, hence him being an antagonist in the film.
The only possible wrinkle in all of it stems from the ending to Marvel’s What If…? At the end of the first season, Strange Supreme appeared to have a change of heart and worked with Uatu the Watcher and the Guardians of the Multiverse to fight Infinity Ultron, an alternate version of Ultron who destroyed the world and was poised to do the same to the multiverse after obtaining his reality’s Infinity Stones. Not only was Strange Supreme vital to their victory, but when an alternate Killmonger and Arnim Zola start fighting over the Infinity Stones, he helps seal them in a pocket dimension to keep them away from the multiverse and agrees to watch over them.
The series ended on the idea of Strange Supreme being a heroic figure once more, but it’s possible that his return to heroism wasn’t genuine and he only worked with Uatu because of the threat Infinity Ultron posed; going along with that, his duty to watch over Killmonger and Zola wouldn’t preclude him from having his own personal agenda. Additionally, considering how the multiverse is in play, it could be that the Strange Supreme featured in the trailer is another version of Strange Supreme who wasn’t involved with the Guardians of the Multiverse and never got a chance at redemption. Whatever the case, assuming that there isn’t a misdirect, Doctor Strange in the Multiverse of Madness is clearly going to have an evil version of the titular character as one of its villains.
