User blog comment:Kajune/Why did Lilith want to kill Sam?/@comment-26982641-20160403064508/@comment-24924499-20160403164544

I don't think we know that for sure. I mean, why else would she have fled when Sam attempted to stab her with it at the end of season 3? From what I've gathered, the knife only kills demons when used fatally, though its effects have been somewhat inconsistent. For example, Meg and Ruby, both mere black-eyed demons, were sliced and mutilated with the knife and it didn't kill them. And when Castiel stabbed Alastair with it, he examined the spot the knife hit (near heart) and said "Almost". Had the knife hit his heart, he would've died, I think, since the wound would've been as fatal as slitting his throat or stabbing him to the head. But like I said, the knife is inconsistent, since sometimes demons die from a single stab, while other demons of similar rank can be sliced and carved with it and it doesn't kill them.