Captain America: Hail Hydra
Captain America was created by a couple of Jewish men who needed an imaginary hero to go and punch Hitler in the face. Those men, Jacob Kurtzberg, later called Jack Kirby and Joseph Simon, received death threats for creating him from their fellow Americans. At the time there were fairly even splits between Americans who wanted to jump into the Second World War on the side of the Allies, those who wanted to remain uninvolved and those who thought that this Hitler fellow had some really good ideas, particularly on the subject of Jewish people.
Within the world of Marvel's comics there is an evil organisation known as Hydra, an offshoot from the Nazi party that could persist after the end of the Second World War so that Captain America and his allies could have an ongoing battle against the forces of Fascism without having to leap around real world enemies that the editors of the time couldn't be sure that history wouldn't vindicate. Not long after that the Captain was put on ice, too much of a political hot potato. It was hard to put the hero of one war into the next one where he might be fighting former allies and friends.
Fast forward to the present day, Captain America is back in action and the writers have decided that they need to add a twist to their storyline to perk things up in the inevitable sales slump following the release of the latest film. They have killed him and brought him back from the dead. They have removed his super powers, made him an old man and then brought him back from that as well. They have had him hand off his mantle to the next generation and snatch it back more times than can be counted. Something drastic was required.
Captain America is now an undercover agent of Hydra, the Nazi organisation that he has thwarted countless times since his creation. Despite the fact that is runs contrary to everything that the character has ever stood for, both in the world of the comics and without. Despite there being a huge number of logical faults and plot holes created by this latest twist. The character has literally gone to war with his best friends to defend civil liberties, his mind has been read by telepaths who consider him to be the moral centre of the human race and Thor has a magical evil detecting hammer that considers the Captain to be a pretty good guy.
Turning a character who has symbolically stood up against fascism and anti-semitism into a Nazi is morally repugnant. The timing of the twist happened to land at around the same moment fans of Captain America were campaigning on Twitter to give the character a boyfriend. The implicit message that Marvel were sending out with that timing was that it was more acceptable for the living symbol of America to be a Nazi than for him to be gay.
Even assuming that you can move past all of that, and that is a big assumption, there is no way that this latest plot twist can work out well for Marvel. They either permanently destroy one of their most beloved characters or the entire furore is for nothing. If this character change is permanent the character has betrayed the most basic tenets of his personality. If it is explained away by mind control, an evil serum, an alien clone or any of the usual comic book shenanigans then the narrative that they are trying to construct is betrayed instead. Either way, the readers and fans of Marvel lose and Marvel loses readers and fans.