One possible motive could be revenge. Maybe Marinette has something that Felix wants, or he blames her for something that happened to him or his family. For example, if Marinette unknowingly got in the way of a business deal Felix was involved in, he might kidnap her to get back at her.
Marinette could use her wits. If she's tied up, she might be able to find a sharp object nearby to cut the ropes. For example, if she's in a room with some old furniture, there could be a nail or a broken piece of wood that she can use.
One possible motive could be revenge. Obito might feel that Kakashi has wronged him in some way in the past, perhaps related to Rin's death. So he kidnaps Kakashi to make him suffer.
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.
Perhaps Bellamy is being manipulated. There could be a third party who has some kind of control over him, either through blackmail or mind - control. This third party wants Clarke out of the picture for their own gain, so they force Bellamy to kidnap her without him fully understanding the implications.
It could also be a case of miscommunication. Sirius might think that Regulus has some crucial information that could help in the fight against Voldemort. So, in his haste and desperation, he kidnaps Regulus to extract that information, not realizing that there are other ways to go about it.
One possible motive could be that in the fanfiction, Adrien is being controlled by some dark magic or an evil force. This external influence makes him act against his true nature and kidnap Marinette. For example, perhaps a villain has cast a spell on him to use him as a tool to get to Marinette, who might have some special power or knowledge that the villain desires.
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 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.