Verified answer

No....but you can ignore the questline.....though I wouldn't recommend it as they provide the most interesting guild missions.

You did have a choice lol. If you kill Astrid instead of one of the three other victims, you get a quest to wipe out the Brotherhood instead of joining them.

Anyway, what everyone else has said is true. You can't leave, and I believe all the other people in the guild are set as essential so you can't kill them. Just finish the questline so you can get it out of your journal, or leave it to rot there.

Unfortunately, on the consoles u can't manipulate the quest stages like u can on the PC. So, without reverting to a save from before u killed the orphanage witchwoman, u are stuck with the Dark Brotherhood in your journal.

U can just ignore anything to do with the Dark Brotherhood and it won't affect the rest of the game elements. U will just have the journal entry for the remainder of the game.

No, you can´t, however you can actually load your previous game save (if you do have one) when Astrid asks you to kill one of the three NPCs, attack Astrid instead and the quest "Destroy The Dark Brotherhood" will begin.

Pretty sure that it isn't possible to leave, however if you are really desperate try killing everyone in the guild. Still pretty sure if you try that though, that the people will just get knocked out and stay alive.