The witches' prophecy that he would be king made him decide to leave it to "chance," but Duncan's announcement that Malcolm was to be his heir made Macbeth realize that he would have to take a course of action for the prophecies to come true.
Macbeth later told Lady Macbeth about this and she just wanted to help him and do whatever she could for him, so that he would be happy and be King.
She was also excited about becoming a Queen so she pushed Macbeth forward and did not let him back down from doing what he said he'd do.
Macbeth started as a courageous and brave general who loved his wife very much.
But because of the faults that must accompany every tragic hero, he was led to his ruin by his overwhelming ambition, superstition and moral cowardice.
Macbeth listened to the witches’ prophecies that said he would become King.
Macbeth did not want to wait any longer and he thought the only way to become King was to kill the present King; Duncan.
After the murder of Duncan, Macbeth sinks into continuous moral degradation.
He was in a savage frenzy when he planned the murder of Banquo and Macduff's family.
The three points which contribute greatly to Macbeth’s degeneration are the prophecy which was told to him by the witches, Lady Macbeth influenced and manipulated Macbeth’s judgment, and finally Macbeth’s long time ambition which drove his desire to be king. "Lady Macbeth convinced her husband to murder Duncan by putting his manhood and courage at stake, "When you durst do it, then you were a man;/And to be more than what you were, you would be so much more the man" (I,7 )As Macbeth started degrading he lost some bravery (IV, 1, "That I may tell pale-hearted fear it lies").
Although he was so far courageous and brave and he is seen as the hero at the beginning of the play, his sky high ambition causes his damnation. In his fight with Macduff, some of his old courage and strength returned.