A Good Man is Hard to Find
The grandmother is the central character in the story. She's the focus of the narrative, the character whose reactions we hear the most about, as well as the only character who's head we get inside (although not too deeply). It's also the grandmother who confronts "evil" in the form of The Misfit. Whether the grandmother is herself a "good" or even a likable protagonist is another question. If you view the grandmother as a conniving manipulator who gets caught in her final act of manipulation, then you might have a hard time identifying with her. On the other hand, if you agree that the grandmother has some kind of "awakening" at the last moment, then her role as protagonist is all the more important. In this light, the story becomes centered on her transformation in the face of an extreme situation.