Some movies thrive on outing the good guy as actually being the bad guy toward the end of the film. In the MCU, it is fairly clear who’s good and who’s bad, but a dark theory aims to turn that ideology on its head.
Doctor Strange, sorcerer and primary protector of Earth, is said to have been behind the jaw-dropping event that saw Thanos snapping his fingers in the Infinity Gauntlet and eliminating half the universe.
According to this theory, Stephen Strange wanted to rid Earth of Thanos as well as Iron Man, Vision, Hulk, Thor, and the Infinity Stones. Strange saw them all as threats to the existence of the planet. But in the end, he succeeded only in getting rid of Vision and Iron Man.
The theory is greatly expanded when considering the rationale behind the demise of each hero and villain (according to Strange). This includes Tony Stark being reckless, Vision being an abomination, Hulk being a loose cannon, and Thor being unable to save his own planet and therefore unable to save Earth. Furthermore, the Infinity Stones posed a threat to reality. By ridding Earth of them, reality would be safe.
It has even been theorized that Doctor Strange didn’t really disappear after the snap. Instead, he faked his own vanishing.