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.
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 great Bucky x Steve fanfic is 'A New Beginning'. It delves deep into their relationship post - Avengers events, exploring how they reconnect and heal together. The characterizations are on point, making it a very engaging read.
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.
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.
In some fanfics, Bucky might be written to hate Steve due to misinterpretations or plot twists. Maybe the writer wants to explore a darker, more conflict - ridden relationship. It could be that Bucky has been brainwashed in the fanfic and his altered mind now sees Steve as an enemy, or there could be some past event that was misconstrued and led to this hatred.
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 plot development could be that Steve realizes his strength has accidentally put Bucky in a dangerous situation. He quickly tries to lift his weight off Bucky and helps him up. Then, they might have an emotional conversation about Steve's powers and how they need to be more careful around each other.