One literary device that supports the theme of good vs. evil in “A Good Man Is Hard to Find” is the simile which compares the grandmother to an innocent child.
We are told that “her legs crossed under her like a child's,” giving us the impression that, despite her advanced years, the old lady is childlike when it comes to apprehending evil as personified by the Misfit.
The grandmother may not be good in the purest sense of the word, but compared to the Misfit, she's the epitome of all that's sweet and wholesome. In due course, the grandmother's naivety in dealing with evil will see her make the fatal mistake of treating the Misfit as less of a threat than he actually is, thus precipitating her own murder.
It's when the fundamentally childlike grandmother, a naive young girl beneath all the wrinkles and her acerbic, uncompromising exterior, reaches out to the Misfit as if he were her own child that she seals her own fate. Unable to deal with someone treating him like a human being, the Misfit responds by killing the grandmother.