Be warned spoilers below if you haven't watched the movie.
The Killer in The Vanishing (1988, orig.: Spoorloos), his primary motivation as per my understanding was to gauge himself to see if he could perform an act so evil compared to his act of saving the person who was drowning and because of that he decided to kill the girl whom he kidnaps from the gas station in the beginning of the movie.
What intrigues me is why does he have to kill the girl's boyfriend 3 years later there was no evidence shown in the movie which gives the inkling that killer could be psychotic or a serial killer although it was shown he clearly was a sociopath.
He could've told the girl's boyfriend about the fate she suffered and let him leave because there was no evidence connecting the killer to the missing girl. I couldn't think of any reason why he has to make sure the boyfriend suffered the same fate as well.
My question is what is his primary motive in killing the girl's boyfriend?