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.
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.
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.
One reason could be to explore the extreme consequences of their ideological differences. In the official story, they had disputes over things like the Sokovia Accords. In fanfiction, this could be pushed to the limit, leading to Tony taking Steve's life.
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.
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.
Perhaps there's a power struggle over a new technology or a powerful artifact. Tony wants to use it for what he believes is the greater good, but Steve opposes him. Their clash escalates to a point where Tony, in a fit of rage or desperation, kills Steve. Or maybe it's a situation where Steve discovers some dark secret about Tony's business dealings and Tony kills him to keep it quiet.
It's a complex issue. In the context of fanfiction, it's fictional and can explore different themes. However, from a moral standpoint in the real world, it's disturbing as both are heroic characters. But in the realm of creative writing, it can be a way to explore 'what - if' scenarios, like exploring deep - seated conflicts between the two that could hypothetically lead to such an extreme situation.
Perhaps in a post - Endgame world, Tony didn't die but lost his memory. Steve takes it upon himself to help Tony regain his memories. Along the way, they re - explore their relationship, the Avengers, and all the events that led to that point. Steve shows Tony videos and tells stories, and slowly Tony starts to remember, leading to a renewal of their friendship and a new start for the Avengers.
Yes, there are likely fanfictions out there where Tony kills Thanos. The Marvel universe has a huge fan base, and fans often create alternative storylines. Some might explore this scenario to give a different take on the Infinity War and Endgame stories.