Perhaps O’Connor’s own words can help you come up with a subtitle. In her letters, O’Connor wrote:
“I am mighty tired of reading reviews that call “A Good Man” brutal and sarcastic. The stories are hard but they are hard because there is nothing harder or less sentimental than Christian realism.” (from The Habit of Being, page 90).
One of your subtopics could be that O’Connor uses violence to achieve Christian realism in her stories. The violence often leads the characters to profound moments of truth, even if it is right before they die (such as the grandmother and Mrs. May).
She also explained that because she was writing in a post-Christian world (unlike other writers who focused on religion such as Dante and Milton), most of her readers did not have the Christian world view, did not even believe in God, and therefore she was forced to use violence to shock her audiences into paying attention. So another subtopic could be that the violence is used for shock effect to achieve her themes.
Thirdly, O’Connor presented Jesus in her novel, The Violent Bear it Away, as a “bleeding stinking mad” savior – a savior who turned the world upside down, a God who came into the world in terrifying glory, not like a meek little lamb. To follow Christ took strength, she believed, and it was not for the meek. People thought they could get by in life without any faith. As the Misfit says, he was doing just fine by himself until Jesus came and messed everything up. The Misfit is incorrect, however. He is not doing just fine by himself – without faith, he has become a psychopath. O’Connor wrote that “What people don’t realize is how much religion costs. They think faith is a big electric blanket, when of course it is the cross” (from The Habit of Being, page 354). Christ’s death on the cross was costly, bloody and violent, and O’Connor tried to show this with her form of Christian realism. So a third sub-topic could be that O’Connor used violence to show a different side of Christianity, a side that was difficult and costly for its savior.
I have given you a general link to information on eNotes about Flannery O'Connor, but we have summaries of many of her short stories, including the ones you mention, so click on the individual story links and you can get even more information that will help you with your subtopics.