Perhaps Sasuke has some sort of vendetta against Sakura that we don't know about in this fanfiction. It could be related to some past event that only he remembers or misinterprets. Or maybe he wants to test Naruto's strength and loyalty through Sakura.
One possible motive could be to use Sakura as bait. Maybe Sasuke is trying to lure Naruto to a certain location. He knows Naruto cares deeply about Sakura, so he figures Naruto will come after her.
One possible motive could be that Sasuke is being controlled by some dark force. Maybe an enemy has a hold over him and is using him to kidnap Sakura as a way to get to Naruto or the Hidden Leaf Village. Another reason might be a misunderstanding on Sasuke's part. He could think that Sakura has some information that is crucial to his personal vendetta and decides to take her by force to get that information.
Sasuke might want to get some information from Naruko. Maybe he suspects that she knows something about a threat to the Uchiha clan that he is trying to uncover.
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.
One possible motive could be that Sakura is brainwashed. Maybe an enemy uses some kind of mind - control jutsu on her and she becomes a puppet to kill Sasuke. Another could be extreme jealousy. If she felt that Sasuke was slipping away from her in some way, in a very dark and twisted version of the story, it could lead to her killing him.
A possible motive could be a creative exploration of the 'what - if' scenarios. Maybe the writer wants to see how the story would unfold if Sasuke completely lost his way. Another motive could be to create a sense of high - stakes drama. Since Sakura is an important character, her death at Sasuke's hands would be a huge shock.
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 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.
Perhaps it's about love in a twisted way. Sebastian might be obsessed with Clary and think that by kidnapping her, he can make her his. In some fanfictions, characters with a warped sense of love do extreme things like this. He could also be trying to use Clary as a pawn in a larger scheme against the Shadowhunters. By having her in his control, he can manipulate the situation to his advantage.