Maybe in the fanfiction, Naruto is being controlled by some dark magic or a powerful enemy. This could force him to kidnap Lemon against his will.
One possibility could be that Naruto believes Lemon has some crucial information that could save the village or his friends. So, in a desperate attempt, he kidnaps her to get that information. Another could be that there's been some kind of misunderstanding. For example, Naruto might think Lemon is a threat to the village when she actually isn't.
It could be that Naruto is trying to protect Lemon from something or someone in a very extreme way. He might think that by kidnapping her, he can keep her safe from a greater danger that he knows about. Or perhaps there's a prophecy or some kind of pre - determined fate in the fanfiction world that Naruto believes requires him to kidnap Lemon for the greater good, even if it goes against his normal moral code.
One possible motive could be protection. Annabeth might think that there is a threat to Percy that she can only handle by taking him away from the situation. For example, if there's a prophecy that puts Percy in danger and she believes she can keep him safe in a hidden location.
Maybe he wants to use Ichigo as a bargaining chip. If there are other forces or characters in the fanfiction world that Kisshu is in conflict with, he could kidnap Ichigo to gain an advantage over them. For example, if there are rival groups vying for power or resources, Ichigo might be valuable to them in some way, and Kisshu could use her to negotiate. Another motive could be that he believes Ichigo has some sort of special power or knowledge that he wants to extract from her, and kidnapping is his way of getting close enough to do so.
Another motive could be related to some sort of power play. Eric might want to use Sookie as a bargaining chip. Since Sookie has her special abilities, other powerful entities in the fanfiction world might be interested in her. Eric could kidnap her to gain an advantage over his rivals.
One possible motive could be that in the fanfiction's plot, Illya might be influenced by some dark magic or external force that compels her to kidnap Shirou. For example, if there is an evil entity controlling her actions, it could use her as a tool to get to Shirou for some unknown, perhaps sinister purpose related to his powers or his connection to other characters in the story.
It could also be for power. In the fanfiction world, Sebastian might think that by kidnapping Kurt, he can gain some sort of power over the New Directions or the McKinley High glee club. He might want to use Kurt as a bargaining chip to get what he wants from them.
One possible motive could be a misunderstanding. Maybe Steve believes Tony has done something that endangers the team or the world, but he's misinformed. Another could be that Steve has been mind - controlled. In some fanfictions, external forces take over Steve's mind and make him do things he wouldn't normally do, like kidnapping Tony.
One possible motive could be a misunderstanding. Maybe Naruto discovers some false information that makes him believe Konoha has betrayed him. For example, if he's wrongly led to think that the village elders are planning to harm his friends or his sensei. Another motive could be the influence of a powerful and evil entity that has somehow managed to control Naruto's mind. It could use Naruto's strength to achieve its own goal of destroying Konoha.
One possible motive could be ransom. If the mom is an influencer, the kidnappers might think she has a lot of money or valuable assets, and her family or sponsors would be willing to pay a large sum to get her back.
One possible motive could be that Bill has some sort of vendetta against Wendy. Maybe she has something he wants, like a powerful object or information that could give him an edge in his evil plans. Another reason could be that he wants to use her as a bargaining chip against someone else, perhaps a hero or another character who has been thwarting his plans.
One possible motive could be mind control. Maybe an enemy uses some kind of jutsu to make Naruto lose control and attack Sakura. Another could be a misunderstanding. For example, if Sakura was framed for something really bad, and Naruto in his hot - headedness believes it and attacks her. Or perhaps in a dark, alternate universe version of the Naruto world where the characters' personalities are twisted, Naruto might have a reason to attack her based on that altered personality.