Marethari gave her life for something. It's an abomination who claims that it was to protect Merrill. And Marethari had previously ensured that Merrill would never find any allies within the clan again, which certainly doesn't sound like the act of someone who still cared about Merrill... in addition to throwing her at a varterral, which doesn't sound like someone overprotective.
I see... well, even if we don't take the abomination at its word, there is still the fact that Marethari made repeated attempts to turn Merrill away from blood magic over the course of... 5-6 years? Now, it's true that this was Merrill's decision to make, but what other motivation could she possibly have had for this? Jealousy that Merrill might accomplish something she never did as a Keeper? Tbh I can't think of anything, and even the reason I just gave sounds kind of absurd... I also would not say that Marethari turned the clan against Merrill. She was a blood mage, and all Dalish know what blood mages are... the Keepers are so wary of Fade creatures that they supposedly don't normally work magic involving spirits. Unless Merrill's blood magic was a closely guarded secret, the clan would have found out what she was doing and come to their own conclusions... most likely negative ones. Finally, the varterral quest was supposed to deter her from continuing along this path, since she was invoking a Dalish law that could not be overruled, even by the Keeper. Maybe she figured Merrill would be too afraid to fight the beast, or maybe she trusted Hawke and company to keep her safe... who knows. I don't think she made this request out of malice, however, hoping that she'd die as a result.