She's basically deuteragonist of the story, if she starts murdering indiscriminately, then there's no reason to care about her story except hoping someone will manage to kill her off soon, which obviously won't happen. Why would I want to watch the adventures of a murderer?
"This monster wants to eat me" found a good way to do protagonist monsters, that still feel dangerous, without including random victims just for the edge (as far as I've seen). And I guess "Parasite" also counts, with the other monsters doing the indiscriminate killings, since deuteragonist is trapped with MC.