I think the only reason this even sounds unique is because the girl was a teen and therefore hadn't learned the art of manipulating men to do evil for her subtly.

Because I've known a handful of those serial killer girls, and they all are always quietly pushing you towards committing crimes of that nature because it turns them on, the same way fujos are always trying to turn their guy friends gay. Since actually finding and then fucking a serial killer is very difficult these days, its probably the next best thing to fulfill their fetish.