Robert Downey Jr.'s role as Doctor Doom in Avengers: Doomsday is one of the most shocking decisions the Marvel Cinematic Universe has ever made, and a new theory might explain why the franchise's biggest star went from hero to villain. Downey's time as Iron Man helped make the MCU Hollywood's most successful franchise. Marvel shocked the world when the studio brought him back to play Doctor Doom instead of Iron Man. He was announced as Victor von Doom, but it is hard to believe that no Tony Stark connection will exist.