Question

Why does the grandmother call the misfit a good man several times during their conversation

Answer 1

First, it is because the grandmother was attempting to compliment the Misfit by calling him a good man and persuade him to stop killing her family members.

Second, it is because the grandmother thought that by calling him a "good man", she would also believe and be convinced that The Misfit really is a good man.

Answer 2

In Flannery O'Conner's "A Good Man is Hard to Find" towards the end of the story, grandmother claims Misfit being a good man because of two assumptions. First explanation is that maybe grandmother wants Misfit not to murder her family members and the second one is related to her own horror and possible escape from the situation. Indeed, she is powerless against the Misfit.