A good question. Have you ever heard the expression "there are no atheists in foxholes"? It means that when things are tough, especially when people are in a crisis situation, people rise to the occasion. Think about how ordinary people became heroes on 9/11.
The Misfit is referring to how most of us live our lives in a petty, selfish fashion day to day. That's certainly true of the grandmother, but when he points a gun at her, she tries to leap to a higher moral level. She talks about Jesus and giving her money away, but she only does so in an attempt to save her life. If she believes those things, she should be living those beliefs—and she doesn't, at least not as we see her. She's mean and selfish, except when a gun is pointed at her.