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.
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 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 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.
One possible motive could be love. In the fanfiction, female Naruto might be secretly in love with Sasuke and sees kidnapping as a way to keep him close. Another motive could be to prevent Sasuke from seeking revenge. She may think that by kidnapping him, she can stop him from going down a dark path that will lead to his destruction.
The idea of Sasuke kidnapping Sakura in fanfiction is quite interesting. It could be used to explore different aspects of their personalities. Sasuke is often seen as a brooding and complex character. By kidnapping Sakura, he could be showing his possessive side or his insecurities. Sakura, on the other hand, would have to show her strength and resilience in such a difficult situation. She might try to talk some sense into Sasuke or find a way to escape on her own. This kind of plot can create a lot of tension and excitement in the fanfiction.
Perhaps in the fanfic, Sasuke was being pushed to his limits. He was so intent on protecting the village from a powerful threat that he didn't realize Sakura was in a dangerous position. His Sharingan might have malfunctioned in some way, causing him to misinterpret Sakura's movements as hostile. Or, there could be a situation where an enemy was using Sakura as a shield, and Sasuke's attempt to strike the enemy ended up harming Sakura instead, all in a split - second decision.
One possible motive could be jealousy. Grindelwald might be jealous of Dumbledore's power and popularity in the wizarding world. By kidnapping him, he wants to prove that he is the more powerful one.
Shego might kidnap Kim Possible in fanfiction because of jealousy. Maybe Kim always gets the credit and Shego wants to show that she's just as capable. So, she kidnaps Kim to prove a point.
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.