Perhaps in the fanfiction world, Naruto has lost his sanity for some reason. Maybe he has been through a traumatic event that has warped his sense of right and wrong. Hinata, being someone he cares about deeply, becomes a target of his deranged actions. However, this would be a very dark take on the Naruto - Hinata relationship, and often such fanfictions use this extreme situation to then explore how Hinata might be able to bring Naruto back to his senses, or how the other characters in the Naruto universe would react to such a situation.
One possible motive could be his thirst for power. By kidnapping someone with a special ability or knowledge, like a powerful Force - user or a brilliant scientist, he could gain an edge in his quest to rule the galaxy. For example, if he kidnapped a Jedi who knew the location of ancient Sith artifacts, he could use those artifacts to increase his own strength.
Perhaps it's a personal grudge that Gaara has unknowingly held. Maybe there was an incident in the past where Naruto's actions had an indirect negative impact on someone Gaara cares about in Suna. Gaara's actions are often driven by his emotions and his past traumas. He could have bottled up this grudge without realizing it and finally decides to kidnap Naruto to get some sort of revenge or closure.
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.
One possible motive could be his fear of losing her. In the Star Wars universe, Anakin is deeply in love with Padme. If in fanfiction he feels that she is in danger or might be taken away from him by some external force or person, his desperation could lead him to kidnap her in an attempt to keep her safe. For example, if he suspects the Jedi Council of having some secret plan regarding Padme, his unstable state of mind might push him to this extreme action.
Well, Ursula could kidnap Ariel in fanfiction because she wants to take Ariel's place. Think about it. Ariel has a loving family, a great voice, and is adored by many. Ursula, on the other hand, is often shunned. So she might think that by kidnapping Ariel and getting rid of her, she can step into Ariel's life. Maybe she wants to be the one who is loved by Prince Eric. Or perhaps she wants to be the favorite daughter of King Triton. It's all about her twisted desires for a different kind of life.
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.
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.
One possible motive could be power. Damon might see Elena as a pawn in his game to gain more influence in the supernatural world. He could think that by kidnapping her, he can control those who care about her and make them do his bidding.
Well, in some fanfictions, Bill might kidnap Will to gain more power. Maybe Will has some special ability or knowledge that Bill wants to use for his own evil plans.
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.