Revenge might also be a motive. Maybe someone had wronged him in the past, and he decided to kidnap a loved one of that person. He could then use the kidnapped individual as bait or a means to torture the person who had wronged him. For instance, if a Jedi had betrayed him, he might kidnap the Jedi's padawan to get back at him.
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.
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.
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.
One possible motive could be misinformation. Maybe Bucky was misled to believe the OC was a threat to someone he cares about. So, he kidnapped the OC to prevent them from causing harm.
Maybe Dumbledore believes that Harry has some hidden power that is being corrupted by the environment at Hogwarts. So, he kidnaps Harry to a secluded place to train him and help him control this power without any distractions or negative influences from other students or even some of the teachers.
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.
A powerful crime syndicate could be responsible. They see Anakin as a valuable asset. Maybe they think they can sell him to the highest bidder, whether it's the Sith or some other powerful entity in the galaxy. They would use their extensive network of informants and thugs to carry out the kidnapping.
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.