^^^^can't say I remember for certain, but I thought that was just a sword to kill and seal Acathala. If she would behead Angel with it, game over, but just piercing his heart wouldn't do the trick. Course, I'm not sure about that. 🙂
When Buffy says that she killed Angel I agree with the folks that say overall:
Buffy went to kill Angelus and if necessary re-seal Acathala before he could draw in his first breath, she had no knowledge of Willow trying to re soul Angelus and bring back Angel.
Suprise! He gets resouled, but only after Acathala starts to stir. The only way to undo this is by using Angels Blood, so she pins them direct route, heart to heart, together. She thinks this may kill him, but its something she has to do as it is the fastest way to seal the portal.
She stabs him but he does'nt die, instead Angel gets sucked into hell...I guess his blood took just that long to seal Acathala, or maybe thats what happens to the conjurer that messes with Acathala and fails to deliver? I dunno.
I wonder if she really knew it would kill him, she never talks about it like it was a slay- and in Angels case that would be like a murder for her, so much as a consequence of her actions which she took because of her duty and role as a hero saving the world. I mean she says she killed him, not murdered him. Probably just a technicality, but the sword through his heart I don't think would kill him. Acathala taking him to hell certainly would. Ican't remember if Acathala taking the creature that awoke him to hell is a part of prophecy and such when dealing with resealing Acathala, I'm not sure but I don't think Buffy saw that one coming.
just my yammerings based off of some of everbody elses ideas.