Luna did seem to try to convince her dad not to hurt Mary before being set in her decision, but when he interrupted her and made it clear he was for sure on the side of killing Mary, Luna gave up any hope of convincing the townspeople to back down. She ultimately chose Mary over everyone else. It did seem to me like the story hinted at the possibility of Luna having some mental health issues, given she did live in extreme isolation due to her father's preferred lifestyle.
Though the ending does also hint that some level of vampire charm may have been at work. I suppose the question the story poses is "Did a monster make a human do something monstrous, or were aspects of human nature monstrous to begin with?" I generally think it's the latter, given how the story pointed out how violent the militia were out of fear, even disregarding Luna's safety if it meant taking a shot at Mary.