I agree. I don't think Mama was dense at all. In the novel she tells Kay that she goes to mass each morning to pray for her husband so he "don't go down there." She knew her husband.

But this thing about Mike: he was, to use someone's term, a sociopath. He used murder as a method to achieve his ends as he perceived those ends. That perception helped to rationalize his actions. Yes, at some point the immorality of what he had done may have broken through to him. Call it lucidity if you will. But as Tom explained to Kay, you can't forgive treachery. Well, why can't you? Not forgiving is a choice and Micahel chose just about everything he did in his life.