So what if you instead decide to slay the dark brohood themselves, should there still be a way to join them after that? Players shouldn't be able to have everything, do everything, see everything in one playthrough. If you can your not playing much of a role in a living, breathing world, the game is blandly designed to pander to you, the world is clearly revolves around you.
Of course, I only argue unforseen consequences as vital as rare occurrences, just to spice things up and appear more organic rather than designed. For the most part you do want to stick to Sawyer's words of wisdom above though.