That's an interesting question. I'm not sure I would have thought of it this way. Because of his moral concerns, he wanted to put fears over the nature of his crime to rest. That is to say, he wanted to get past the sense that this was wrong due his being Duncan's subject and kinsman. More directly, though, he wanted to put to sleep the doubts about being man enough (or fierce/brave enough) that his wife had stirred in his uneasy brain.