Some fans would be extremely shocked and disappointed. Steve and Tony are two beloved characters, and their relationship is complex and interesting. Killing one of them off in such a way would be a huge departure from what many expect.
Many fans would likely be divided. Some would condemn it as a violation of the essence of the characters. However, others might appreciate the boldness of the concept. They could start in - depth discussions about the motives and consequences within the fanfiction, debating whether it's a valid exploration of the characters' arcs or just a shock - value gimmick.
One possibility is that there could be a huge ideological rift between them. Maybe Steve discovers some dark secret of Tony's that he believes endangers the world in a way that can't be resolved through normal means, leading to the extreme act of killing him.
Some fans would be really shocked and sad. Kakashi is a very popular character, and his death at the hands of Obito would be a huge blow to many. They might feel angry towards the author for creating such a tragic scenario.
On the other hand, some fans might be interested in seeing how the author develops the story from such a controversial starting point. They could be curious about how Pepper will respond and if there's a chance for redemption for Tony.
Some writers might want to explore dark and extreme scenarios. It's a way to break the norms of the usual heroic storylines. They may be interested in seeing how the characters and the Marvel universe would react to such a tragic event.
Some fans would be extremely shocked and heartbroken. Dean and Castiel have a very complex and beloved relationship in the fandom, so seeing Castiel kill Dean would go against what many fans hope for in their relationship. They might be very vocal about their disappointment.
Natasha Romanoff would be disappointed. She values the unity among the Avengers and sees Tony and Steve as important pillars of that unity. She might try to mediate at first, but if the divorce is inevitable, she'd be sad to see the team's dynamic change so drastically.
A lot of fans reject this type of fanfiction outright. They feel it disrespects the characters and the values of the original series. The 'Harry Potter' series is about love, friendship, and fighting against evil, and having Harry kill Hermione completely goes against those ideals.
Some fans are horrified. They love the characters as they are in the original series and any deviation that involves such a violent act against a main character like Sakura is unacceptable to them.
Well, it's not super common but it does exist. There are different reasons for this kind of plot. Maybe the writer wants to explore the consequences of a breakdown in their relationship. They had their differences in the movies, like in 'Civil War', and some fanfic writers take it to an extreme by having Tony kill Steve to show how far things can go wrong when they can't resolve their issues.
One possible plot could be a misunderstanding that leads Steve to believe Tony has done something extremely harmful to the team or innocents. For example, if there was false evidence suggesting Tony was secretly working with an enemy. Another plot might involve a moral disagreement that gets out of hand. Maybe they have different views on how to handle a new threat and Steve, in a moment of extreme passion, makes a terrible mistake.