Ross tells MacBeth this when he says "And, for an earnest of greater honour, He bade me, from him, call thee Thane of Cawdor; In which addition, hail, most worthy thane! For it is thine."(1.iii.109-112) This news solidifies the first half of the Witches prophecy about MacBeth and also brews ambitious thoughts in MacBeth to become King. MacBeth thinks that because he became the Thane of Cawdor like the Witches said, he will also become King. He is met with disappointment however when Duncan names the heir to the throne as his son Malcolm. The result of this is the beginning of MacBeth's corruption. Not satisfied with the amount of power he has obtained as the Thane of Cawdor, MacBeth plans on killing Duncan in order to gain his power. His plans are disrupted when he starts getting second thoughts about the murder. He expresses his moral confusion when he says "I have no spur to prick the sides of my intent, but only vaulting ambition, which o"erleaps itself and falls on the other"(1.vii.25-28). MacBeth is saying that he wants to be King but there isn't much driving him to commit murder. This shows that even though MacBeth considered killing Duncan in order to gain power, he still not totally corrupt and has some humanity left in him.
The little glimmer of hope that MacBeth had of staying a decent and honest person vanished when MacBeth made up his mind to go through with killing Duncan. MacBeth has now decided that he will kill King Duncan, mainly in part to his wife's influence over him. After he kills MacBeth he is worried that he will not be able to clean his hands of Duncan's blood when he says "Will all great Neptune's ocean wash this blood clean from my hand? No; this my hand will rather the multitudinous seas incarnadine, making the green one red."(2.ii.77-80) This shows that MacBeth has indeed killed Duncan and that the King's blood is on his hands. The outcome of Duncans murder was exactly what MacBeth wanted, he was named the new King of Scotland.
