Why does lady Macbeth act so kindly to duncan?

Before the murder, she thinks of it as a great idea, the only way she can see to becoming queen. She doesn't see beyond that. But there is also an almost sexual excitement to her anticipation. When she prays to her "murdering ministers" she begs them to come to her woman's breasts and take her milk for gall. When she perusades Macbeth to proceed against his better judgement, she puts his sexual identity on the line: "When you durst do it, then you were a man." And as the act is being committed, she says "That which hath made them drunk hath made me bold. What hath quenched them hath given me fire."

But that is the climax, so to speak, and after it is done there is no satisfaction for her. She has no joy in being queen. Macbeth becomes someone she doesn't know. And the smell of the blood never leaves her, she is saddled with guilt and misery.