I don't understand why people think Merrill is an idiot, she has proven time and time again that she is smart, cautious and trustworthy.
"Anders, there's no such thing as a good spirit. There never was. All spirits are dangerous, I understood that. I'm sorry that you didn't."
Merrill isn't some child toying with things, she gets it. Everyone treats her as a child, everyone treated her like she is gonna go rogue and that she doesn't understand the risks she is taking. She does, she does far better than Anders or Marethari.
Merrill is as smart as she is resourceful, studying the mirror, researching it, cleansing it with the only thing she has. Merrill moved from Sundermount where the demon's influence was strong and when returning back she asked Hawke and the others to come with her in case something goes wrong, that maybe she may turn into an abomination, she has someone to kill her. She thinks ahead, unlike Marethari's shortsighted vision.
Marethari endangered her whole clan because she couldn't accept Merrill deciding a different fate, Marethari seriously makes me angry because of how she babies Merrill, when everything was Marethari's fault in the first place. Merrill said the clan should have moved long ago, but no, Marethari had to let her clan linger in a place so dangerous. Marethari never even talked to her clan about what she was going to do, she didn't even tell them to remove themselves to a safer place while she deals with Merrill, or assign a First to the clan incase she dies. She doesn't even think of the well being of her clan, at all.
Marethari's actions can not be blamed on Merrill, Marethari is her own person, how she reacted to Merrill's freedom was not only dumb but reckless. She abandoned her clan, she risked everything, not even thinking of the consequences.
It was Marethari who was the one that fell to the Demon (in my headcanon, I believe the demon was after Marethari all that time) and it was her that abandoned her clan, far more than Merrill.