What are the possible reasons for a 'Tony kills Steve' plot in Avengers fanfiction?

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 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.

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.

Miscommunication. The Avengers could receive wrong information about Steve's actions or intentions, leading them to believe he's a danger and thus hurt 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.

One possible reason could be memory alteration. Maybe some villain has the power to manipulate memories and has erased Steve from the Avengers' minds. This would create an interesting plot where Steve has to find a way to make them remember him again.

Differences in ideology can also play a part. Tony might have some views on how to handle threats or use technology that the other Avengers don't agree with. Maybe he's more willing to take risks with untested tech that could endanger the team or civilians, which would lead to resentment.

One possible reason could be Steve's strong sense of loyalty to Bucky. Maybe he feels that he owes Bucky so much after all they've been through. He might be so focused on helping Bucky or being there for him that he unconsciously ignores Tony.

One reason could be Steve's long - standing friendship with Bucky. They have a deep history together, and in the fanfiction, that bond might be portrayed as stronger than his relationship with Tony. Maybe Steve feels a sense of loyalty to Bucky that he can't ignore.

In a 'Tony kills Thanos' fanfiction, the plot could start with Tony being haunted by his previous failures against Thanos. He decides to go on a solo mission to study Thanos more closely. He discovers a way to disrupt Thanos' connection to the Infinity Stones. With this knowledge, he devises a plan. He lures Thanos into a trap where he has set up a device that can nullify the power of the Stones. Then, in a tense battle, Tony uses his own advanced technology and combat skills to fight Thanos without the Stones' advantage and finally manages to kill him. This could be a story of redemption for Tony, showing his determination and ingenuity in the face of seemingly insurmountable odds.