As a few other people have touched on, Merrill is not stupid (and, seriously, read that link). She’s a legitimate, subtle character with her own traits, flawed, wonderful and mundane. She isn’t an idiot, she isn’t just some sweet little airhead, and perhaps most importantly, Merrill doesn’t…
I think these are all very good points, especially this:
…Marethari intervenes in what is often touted as a great act of “love,” but is actually a passive-aggressive, needless self-sacrifice. By refusing to believe that Merrill had planned things out, knew the risks and could possibly do something she was raised to do (learn everything one can, even the dangerous things) (and do you think she came up with that on her own? someone taught it to her), Marethari not only succeeded in creating a psychologically crushing event, but engineering a scenario where either Merrill has to be “taken care of” by someone else, again, or has to slaughter her entire clan to survive.
Things may or may not have worked out better if Merrill had been left to proceed with her plan, but by repeatedly denying her the opportunity to do so, I feel that Marethari essentially tries to keep Merrill as a dependent child.