Just replayed the Heaven's Feel route... I've played through this before, but every time I play this route, I would be too scared to sleep at night.

Case in point, whatever Kurumi did to those hapless would-be-rapists, the Shadow in Fate Stay/Night did it first.
[+] Spoiler
http://fsn.seorinwastaken.com/videos/breedingbox.flv

And yeah, I do know who that thing really is... personally I think Matou Sakura is actually waaay scarier than any of the yanderes out there.