The Question and Answer section for Macbeth is a great resource to ask questions, find answers, and discuss the novel.
He considers that if he became Thane of Cawdor by chance, he would become king by chance.
The doctor says L. Macbeth is mentally ill and is beyond his practice. He says she needs a priest or God instead of him. More needs she the divine than the physician. God, God forgive us all!
This is a pretty huge question for this short answer space. I suggest you do a separate search with this question and you will find a lot of information.