In some fanfics, a motive could be a deep - seated jealousy. Suppose Steve gets all the glory and recognition while Bucky feels left in the shadows. This could build up resentment over time and lead to a situation where Bucky snaps and kills Steve. However, this is a very different take from their normal relationship in the official stories and really plays with the emotions and expectations of the readers.
One possible motive could be that Bucky is being controlled against his will. In the Marvel universe, he has been brainwashed before. Maybe the people who once controlled him have regained their hold and made him target Steve for some strategic reason, like using Steve as a hostage to gain power or information.
Perhaps in the fanfic, Sasuke was being pushed to his limits. He was so intent on protecting the village from a powerful threat that he didn't realize Sakura was in a dangerous position. His Sharingan might have malfunctioned in some way, causing him to misinterpret Sakura's movements as hostile. Or, there could be a situation where an enemy was using Sakura as a shield, and Sasuke's attempt to strike the enemy ended up harming Sakura instead, all in a split - second decision.
A great Steve Bucky fanfic often has a strong focus on their relationship. It could be their friendship or something more, but it should be well - developed. For example, it might show how they support each other through tough times. Good character development is also key. The fic should stay true to the personalities of Steve and Bucky as we know them from the source material.
Another common theme is loyalty. They are extremely loyal to each other. In some fanfics, they'll go to great lengths to protect one another, even if it means going against the whole world. It's a very powerful aspect of their relationship that writers often explore.
On the other hand, Steve might be more understanding than expected. He knows about Bucky's past traumas and could suspect that something else is at play. So, instead of immediately resisting, he might play along for a while, waiting for the right moment to figure out what's really going on and how to help Bucky.
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 possible motive could be a misunderstanding. Maybe Steve believes Tony has done something that endangers the team or the world, but he's misinformed. Another could be that Steve has been mind - controlled. In some fanfictions, external forces take over Steve's mind and make him do things he wouldn't normally do, like kidnapping Tony.
One possible motive could be misinformation. Maybe Bucky was misled to believe the OC was a threat to someone he cares about. So, he kidnapped the OC to prevent them from causing harm.
One common reason could be jealousy. Maybe in the fanfic, Steve has something that Bucky wants, like a certain power or a relationship with someone. Another reason might be that Bucky has been manipulated by a villain in the story to turn against Steve.
One common theme is friendship. Bucky and Steve have a long - standing friendship, and the addition of the Reader often explores how new friendships are formed and how they all support each other. For example, in many fanfics, they might help the Reader deal with personal problems while strengthening their own bond.