The selfish and mischievous act of the grandmother's in "A Good Man Is Hard to Find" occurs when she manipulates her two grandchildren into whining so that Bailey feels he has no choice but to turn off onto a dirt road in search of an old plantation.
The grandmother wants to see this plantation, which she remembers from years ago, but she knows Bailey won't listen to her, as she has little clout in the family. Therefore, she builds up the plantation to the children by saying that “there was a secret panel in this house.”
She lies; the angry Bailey gives in, turns down the dirt road, and then they run into the Misfit and his gang. They can't escape, because the grandmother has also mischievously hidden her cat in a basket, and it springs out and lands on Bailey's head. He is so startled that he flips the car into a ditch.
However, it is not enough in a thesis statement to say that the grandmother's mischievous and selfish act caused the family to be killed by the Misfit. That's a fact, and a thesis must defend an opinion. You could argue that while the grandmother did a selfish thing, the family was at fault, too, because of the rude and dismissive way they treated her, causing her to be manipulative. You would then need to find examples of them being rude and dismissive to her, driving her to become deceptive. Or you might argue that although her bad act cost the lives of her family, it redeemed her, showing that God acts in mysterious ways. Again, you would have to find quotes that show the redemption, such as when the grandmother sees the Misfit for a moment as her son.