In A Good Man Is Hard to Find, O’Connor explores several themes. Here are the main ones.
Evil can exist where you least expect to find it.
The grandmother does not realize that the Misfit killer is one of her own children. Right before he kills her, she realizes it is her son. She believes in God and does not expect evil in her child.
Actions have consequences.
Granny is not just a harmless, confused woman, and it is clear that she did not treat her children well. She is responsible for how the Misfit turned out. With her selfishness, she also causes her death and that of her entire family by secretly bringing the cat that causes the accident.
The hypocrisy of the religious.
Religion is clearly important to Granny, but she uses it in a self-serving way. She does not care about her children, only herself. She thinks of religion in her final minutes, but only in terms of asking Jesus to save her.