Maybe he wanted to leave some kind of evidence or confession. Or perhaps he was mentally unstable and saw it as a twisted way to make his death known.
The motive could be revenge. Maybe he was getting back at someone and thought this would be the ultimate act of payback. Or it could be a cry for help, a desperate attempt to have his story heard even after his death.
One possibility is that he's in a dangerous situation and records it as a last resort to expose the culprit. Another could be that he's mentally unstable and sees it as some sort of twisted self-expression. Maybe he's involved in a complex conspiracy and this is part of a bigger plan.
It could be due to mental instability. Maybe he has an undiagnosed mental disorder that causes him to act out in such a violent way.
It could be revenge. Maybe one character was wronged and sought justice through killing.
Maybe the husband was mentally unstable and had a sudden burst of violence. Or, there could be a deep-seated jealousy issue related to something the wife did.
One possible motive could be fame. People like to hear about extraordinary things, and having 1000 kids is definitely extraordinary. So, the person involved or the storyteller might be seeking attention.
Perhaps it's a sense of misguided heroism. In their mind, they see the fugitive as someone who is wrongly persecuted, and because of their love, they believe they are 'saving' the fugitive from an unjust fate. So they take the extreme step of abducting them.
Maybe they believed that Hermione had some sort of special power even as a baby that could be both a danger and an asset. By kidnapping her, they could control and train her in a way that would benefit the fight against the Dark side. For example, if she was showing signs of extremely powerful magic at a young age, they may have wanted to ensure she used it for good from the start.
One possible motive could be that Sasuke has been consumed by his thirst for power. In the Naruto universe, Sasuke has often been shown to be willing to sacrifice a lot for power. In this fanfiction set in the Wave arc, perhaps he sees Sakura as an obstacle to his pursuit of power, either because she tries to stop him from following a dark path or he believes she will hold him back in some way.
Well, one possible motive could be revenge. Maybe the guy has some sort of grudge against the man he trapped. Another could be that he's trying to gain something, like information or valuables. For example, if the man trapped has some secret knowledge, the trapper might be trying to force it out of him.
It could be related to a dark secret. Maybe the mama knew something about the killer that the killer didn't want anyone to find out. So, in order to keep that secret buried, the killer took the extreme step of killing her.