It’s the Keeper’s.
If Marethari hadn’t done what she did, Merrill would have been the only one to die. Which would have been sad, yes, but she would have been the only one to pay for her blood magic. The Clan…
I think Merrill bears at least some of the blame. I think one of the themes DA 2 tries to teach us is what happens when you become obsessed with something. 3 of the 4 LIs (Maybe 4 out of 4, but it’s not as obvious with Isabela) become obsessed with an idea and, to some extent, it fucks with their lives.
Merrill- Her obsession with her clan’s history leads to her complete alienation from them. There are a lot of real people and groups who’ve suffered because they clung to the past at the cost of their futures. Yes, Marethari had faults and bears responsibility for what happened, but Merrill isn’t completely guiltless. She is, and I will credit her for this, the most clear-minded of the three obsessive LIs: she comes up with back-up plans and she does recognize the danger of her actions. She even asks that Hawke be there to make sure nothing goes too wrong. However, her disregard for these consequences catches up with her, and in the very ugly and disproportionate way that consequences tend to affect people, both irl and in fiction. What if Hawke couldn’t kill her? What if she became a stronger abomination than could be handled by her friends? She should not be absolved of all responsibility.
Fenris- He and Anders both led very different lives, and their experiences colored their views. They both suffered and witnessed injustices and their views were taken to the extreme. Fenris’ desire for revenge and his obsessive (at this point, instinctive) hatred of mages both cripple him, which is best seen in romances with him. He even risks finding out about his past, finding his family again, for vengeance. He’s so terrified of his past and remembering anything but his hatred that he gives up any chance of a relationship with Hawke until Act 3, when he’s finally started to get closure as far as Danarius is concerned.
Anders- I see a lot of people who’ve excused some of Anders’ obsessiveness because he is supporting a cause. (Never mind that Merrill has a cause, albeit one more limited to the Dalish.) But Anders isn’t just supporting the cause of the mages; after Justice/Vengeance merges with Anders, he nearly becomes his own cause. His obsession just grows deeper and deeper, you can see it progress from being a driving characteristic to the only driving characteristic he possesses. His party dialogue is extremely telling: he’s at least civil with Aveline, Carver and Merrill, he gets along great with Varric, Beth and Isabela, and he and Fenris mostly just bark at each other. As time goes on, his interactions become incredibly venomous: he and Fenris go to the perhaps natural progression of utter hatred, and he becomes so hateful of Merrill it’s actually hard to listen to his dialogue towards her. Even Isabela and Varric remark on it. Isabela and VARRIC, characters who are pretty easy to get along with, and who both have more positive interactions with Anders than anyone else, save a friendly/friendmance Hawke. His obsession with saving his fellow mages ends up killing a lot of them, whether or not Hawke supported the mages or the Templars at the end: with one attack on a neutral party, he starts a continent- (world-?) wide war.
Obsession is a common theme in fiction. It rarely leads to good things; the fact that these three lose so much because of their personal obsessions…I feel that it’s one of the understated points that Bioware tried to make in DA 2.
I feel like a lot of this was probably redundant, and if you feel the same, I apologize for the time you spent reading this. But I hope I made some sort of coherent point.