If you want to get technical, what happens depends on Hawke’s responses to the situations, not Merrill. It’s on Hawke to respond in a way that leads to Merrill stabbing possessed!Marethari or possessed!Marethari stabbing Merrill. It’s on Hawke’s response as to whether the part is forced to fight the Dalish.

Dalish morality is different from the Andrastian morality that followed by most of the game characters. Dalish don’t see spirits as “good” or “evil”; they see them ALL as dangerous, which if you look at what happens when Anders takes Justice is, is a healthier way of looking at things.