One possible motive could be political ambition. Maybe Peter wants to take over the emperor's power and position in the fictional world. If the emperor has a lot of control and influence, Peter might see killing him as a way to gain that for himself.
Peter could be a disgruntled noble in the fanfiction. He might be someone who has been overlooked for promotions or honors by the emperor, and this has led to his resentment and desire to kill the emperor.
One possible motive could be that he has new and important information. For example, he might have found out some insider trading information if it's a business - related meeting. So he interrupts to expose it.
In the fanfiction world, there could be a prophecy or a pre - determined fate that Tony believes in. He may think that Peter's sacrifice is necessary to fulfill this prophecy and save the world from some greater threat related to the Soyl Stone. He might see it as a selfless act in the long run, even though it seems cruel.
Love could be a strong motive. If Tobias has feelings for Tris, he would naturally want to protect her from harm, especially from someone like Peter who might be a bully or a threat.
Peter Pan might be jealous of Henry. Maybe Henry has something that Peter Pan desires, like a special magical ability or an object. So, to get it for himself, he endangers Henry.
One reason could be extreme guilt. Maybe he failed to save someone important and can't bear the burden of that failure. In some fanfictions, if he accidentally caused harm to his loved ones, like Aunt May getting hurt because of his superhero activities, the guilt might be overwhelming.
One possible motive could be jealousy. Maybe Kurt is getting more attention or opportunities in the Glee club that Rachel desperately wants. For example, if Kurt is chosen for a solo that Rachel thought she deserved, it could push her to extreme actions in the fictional world of the fanfiction.
Maybe in the fanfiction's universe, the rules for obtaining the Soul Stone are different. It could be that the Stone demands a sacrifice of someone with a pure heart like Peter, and Tony, in his desperation to stop an even greater evil, makes the ultimate sacrifice. Another reason could be that Tony is being manipulated by some dark force. This force convinces him that killing Peter for the Soul Stone is the right thing to do, and Tony, believing he is doing the greater good, goes through with it.
A plot could involve two Peters from parallel universes. They initially clash because they have different personalities or ways of doing things. For example, one Peter might be more introverted and scientific, while the other is more outgoing and action - oriented. But as they get to know each other, they start to work together to solve a common problem that threatens both of their universes. This could include fighting a super - villain who has found a way to cross between the universes. Another aspect could be that they discover some hidden secrets about their own identities or powers through their encounter with the other Peter.
It could be that Peter has some sort of knowledge or information that the kidnappers want. For example, he might have accidentally witnessed a crime and the criminals want to keep him silent.