I've always found the grandmother's final moments the most chilling part of the story, and the Misfit's reaction to her is the key. She's scrambling, trying to call him a good man, trying to make that personal connection as her family is killed in the woods. And he just sees through it completely. He's not a raving monster; he's eerily calm, almost philosophical. He says that famous line about there being no pleasure but meanness, that life has no real point. He kills her not in a rage, but almost as a conclusion to his own logic. He represents a kind of pure, nihilistic violence that can't be appeased by manners or social niceties. The grandmother might recognize him as 'one of her own children' in that moment, but he's the terrifying end result of a world she helped create, one where grace has to be violently forced upon someone, and even then it might not take.
The story leaves you sitting with his empty, resigned worldview long after you finish it.