One theme deals with violence, but the violence serves a purpose. In this story, the violence is used to finally get the Grandmother to see how hateful her prejudice toward others is (another theme). It's only when the Grandmother's family has been killed that she's able to make any kind of connection with someone else and accept God's grace.
Another theme so aptly shown by the Grandmother is prejudice and intolerance toward others. She believes people of good character are bred by "good families", and she's self-righteous enough to believe she, of course, comes from a good family. This attitude gets her whole family killed as well as herself. She has spent her life concerned with only what she wanted, never thinking of what others needed. This all changes when the Misfit sticks the gun in her face.
For a more complete explanation of themes, go to the site below.