Another possible motive is a sense of betrayal. If he feels betrayed by his friends or the government, for example, he could turn evil. He might think that if they are against him, then he doesn't need to abide by any moral rules. He could then use his resources to strike back at those he believes have wronged him.
Power hunger could be a motive. Tony already has a lot of power with his technology, but if he becomes obsessed with having more and more, it could lead him to evil. He might start to see others as obstacles to his power gain.
Maybe the writer wants to explore an alternate view of Pepper's character. They could be showing that she has flaws and isn't the perfect partner all the time. It could also be a way to introduce new characters into the story in a more impactful way, like the person she cheats with.
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.
The sense of isolation could be a motive. If Naruto never truly felt accepted in Konoha despite his efforts, he might snap. He's always wanted to be recognized, but if that never fully happens, he could turn on the village. For example, if he's constantly passed over for important missions or not given the respect he thinks he deserves as a powerful ninja.
Perhaps the writer wants to explore the nature - vs - nurture concept. Harry grew up in a difficult environment, and in fanfiction, this could be twisted to make him turn to evil. Another reason might be to introduce a more complex villain. Instead of a typical bad guy, having Harry as a secretly evil character adds a new layer of depth to the story.
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.
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.
One possible motive could be a sense of betrayal. Maybe Naruto felt constantly ostracized in Konoha despite his efforts to be accepted, and over time, that bitterness turned into pure hatred. For example, if he was always treated as an outcast by the villagers even after saving them multiple times, it could drive him to destroy the very place that never truly welcomed him.
Power could be a motive. She might want to have control over the vampire world or the humans in Forks. So she starts doing villainous things to gain that power.
One possible motive could be power. Maybe he thinks that by kidnapping the Pink Ranger, he can gain some sort of advantage over the other Rangers or access some hidden power. Another motive might be revenge. If he has some sort of grudge against the Pink Ranger or the whole Ranger team, this could be his way of getting back at them.
One possible motive could be to use Sakura as a bargaining chip. If he has some sort of plan against Naruto or the Hidden Leaf Village, Sakura, being a close comrade of Naruto, could be very useful. He might think that by restraining her, he can force Naruto to come to a certain location or make concessions.