One possible motive could be that the souk stone has some sort of power that Tony is desperate to obtain. Maybe it can grant him ultimate power or fulfill a long - held desire, and Peter is just an obstacle in his way.
In the original storylines, Tony and Peter often have a mentor - mentee relationship. So, in this fanfiction, the act of Tony killing Peter is a huge deviation. It goes against the bond they usually share.
Yes, there might be such a fanfiction out there. Fanfiction often explores dark and alternative storylines. However, it can be quite a controversial and dark concept as Tony and Peter have a mentor - mentee relationship in the mainstream Marvel stories.
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.
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.
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.
Peter could stand up to Tony. By showing his strength and not taking Tony's meanness lying down, he might earn Tony's respect. For instance, if Tony makes a snide comment, Peter could give a smart comeback that makes Tony see him in a new light.
Well, it would likely cause a great rift between those who were close to Peter. For example, if there are other Avengers in the fanfiction, they would be deeply angered and betrayed by Tony's actions. Their trust in him would be shattered, and they might seek revenge or justice for Peter.
I'm not sure specifically as there could be many interpretations. It might be a dark and fictional scenario where Scott has a motive to kill Peter within the context of a fan - created story.
One aspect to consider in 'possessive Tony and Peter fanfiction' is the nature of their relationship. It might explore themes of protection. Tony could be possessive in a way that he wants to keep Peter safe from the various dangers in the Marvel universe, like powerful villains.