I think this quest is the harder in the game, because it doesn't give you choice between destroying this group of crazy people or to sacrifice someone.
In the game, I'm trying to be a good character, but with that, it's destroying everything.
I found a way to make your follower survive, but it doesn't work really well, because yes he survived, but I'm not able to ask him to follow me again. My follower is faendal (yes I know he is ugly) and now he is just repeating what he was saying when you see him in Riverwood for the first time. No option...
Toomorow I will try to make this "glitch" with Lydia to see if it work correctly and if not, I will just skip this mission.
I hope in The Elder Scrolls VI we will have more choice in quests.