Destroy the Dark Brotherhood

So I killed the old hag in the orphanage and the next time I slept I was woken by an assassin offering me a choice of people to kill to make up for steeling their contract. I chose to kill the assassin instead of any of the innocent captives. Now I have a quest to destroy the dark brotherhood.

Makes you think though. Every play though of this game could be completely different depending on the choices you make. Straight away I am wondering what would have happened if I had made a different choice.

P.S I would have used spoiler but I can't find it in the editor

I've read somewhere that in the DB quest you'll finally meet I wonder if it will be possible also to slay her once and for good.