So there's no way to kill him and not aggro the whole camp. That's disappointing. I'm sure come final release that'll be sorted out. Until then, I gave up trying to be stealthy and just killed Minthara first, then Ragzlin and fastr-travelled out after reporting back to Halsin. Now I have to fight my way past all the goblins outside to get back in and I'll finally get into the Underdark in one of my playthroughs.
While I'm on the subject, I really HATE having to deal with all those goblins outside to get back into the sanctuary. It's just a dull, frustrating slog. I've already done all the important stuff in the area, just let me back in. Get rid of thee goblins so I can move on with the story. Halsin's dialogue implies that the goblins are meant to go away with the leaders dead and I sincerely hope that by the time EA is done, they follow through on that.