It could also be a result of some external influence, like a dark magic spell that has clouded Ron's judgment. This spell could be making him act against his normal character and do something as extreme as kidnapping Hermione. However, in the end, he would likely break free from this influence and realize the error of his ways.
One possible motive could be self - defense. Morgana had turned to the dark side and was constantly causing harm to Camelot and its people. Merlin, as the protector of Camelot, might have seen killing her as the only way to stop her reign of terror. For example, she was plotting against Arthur and trying to bring down the kingdom. Merlin's loyalty to Arthur and Camelot could have driven him to this extreme decision.
One possible motive could be to protect Sasuke. Itachi might have seen a great danger approaching Sasuke that he could only avoid by taking him away forcefully. Maybe there was a powerful enemy targeting Sasuke, and Itachi thought that by kidnapping him, he could keep him safe in a hidden location and train him to become stronger.
Maybe Rowle is under the orders of a more powerful dark wizard. In the world of Harry Potter, there are always those higher - ups in the dark side who might want Hermione for her knowledge of magic, especially things like ancient spells and magic theory. So Rowle could be tasked with kidnapping her for that purpose.
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.
Jealousy might be a motive. If Hermione is constantly outshining them in academics or getting more praise from teachers, they could become envious and turn against her.
In some fanfiction scenarios, it could be a matter of different ideals. Suppose there is a new movement in the wizarding world that Ron and Hermione believe in, but Harry opposes it. Their loyalty to this new ideology could be so strong that they see Harry as an obstacle and decide to betray him. It could also be that they are under the influence of a powerful and evil spell that warps their minds and makes them act against Harry without their full awareness.
Another motive could be related to her family. Maybe there was some sort of blood feud or an old grudge against the Grangers. The kidnappers could have thought that by taking Hermione, they could get revenge on her family. In some fanfictions, there are secret magical families that have long - standing rivalries, and Hermione's kidnapping could be part of that.
One possible motive could be revenge. Maybe Draco feels that Ginny or the Weasleys have wronged him or his family in some way. For example, if he blames Ginny for getting Harry Potter on their side all the time which has made things difficult for the Malfoys at Hogwarts. Another motive could be related to power. He might think that by kidnapping Ginny, he can gain some sort of leverage over the Order of the Phoenix or Dumbledore's side. It could also be out of jealousy. If he has some unspoken feelings for Ginny but she's always with Harry, he might kidnap her in a fit of jealous rage.
One possible motive could be that Bakura wants to use Anzu as a bargaining chip. Maybe he's trying to get something from the main characters, like a powerful item or information. Another motive might be related to some sort of revenge. He could think that Anzu is close to someone he has a grudge against, so by kidnapping her, he can cause pain to that person.
One possible motive could be to use Naruto as a bargaining chip. Itachi was a complex character with his own goals related to the safety of the Hidden Leaf Village in some ways. He might have thought that by kidnapping Naruto, he could influence the village's decisions. For example, he could try to stop certain actions the village was planning that he believed would lead to more harm. Another motive could be related to the Nine - Tails within Naruto. Itachi might have wanted to study or control the power of the Nine - Tails for his own purposes, perhaps thinking it could be a way to protect Sasuke in a different manner.