The truth is, you are never responsible for the choices in life someone else makes, unless you have been actively manipulating them/forcing them. And I don't think that's the case with Merrill. Also, that line of thinking goes into victim blaming real fast as well; a child deserves to be abused because they didn't listen, someone obviously got murdered because they were in the wrong space, etc. And I think you should be really careful with that.

Technically I think it's most accurate to say that it's mostly the demons fault in the first place. They are the ones directly being harmful here. Yet people don't consider it something that can take responsibility, which I've always considered very odd. The spirit/demon also has a choice and made it.

However, Marethari is the one who chose to become possessed by the demon and she is a 100% responsible for that choice, no matter her intentions with it or how noble (or not) that choice was. The situation was far from desperate too, Merrill was, as far as I've gathered, pretty safe at that point.