Macbeth goes from hero to tyrannical coward.
At the beginning of the play, Macbeth is a brave and loyal soldier. The sergeant sings his praises to Duncan, who calls him valiant.
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,(20)
Like valor's minion carved out his passage
Till he faced the slave (Act 1, Scene 2)
As it turns out, Macbeth is meeting with the Weird Sisters at about this time. They tell him that he will be promoted, and will become king. Macbeth laughs it off, and even wonders why he is being called Thane of Cawdor, but he clearly is thinking about it. We can tell by his reaction when Duncan names Malcolm his successor.
The Prince of Cumberland! That is a step(55)
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see.(60) (Act 1, Scene 4)
There is no reason for Macbeth to expect to be king, and he has already gotten a big promotion for his service in the war. Yet the witches have planted the idea in his head, and he can’t get it out. When he tells his wife about it, that’s it. There is no turning back. She is convinced that if he wants to be king he needs to take it.
Macbeth does not go from valiant hero to bloodthirsty killer overnight. At first, he does not want to kill Duncan.
He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door (Act 1, Scene 7)
Unfortunately, Macbeth is already becoming unstable. He hallucinates a bloody dagger, and interprets it as guidance to go through with the plan. Yet he is disorganized in the murder, and instantly plagued by fear and doubt. It is only Lady Macbeth’s firm hand and carefully planning that makes him successful.
Macbeth’s doubt turns into fear. He has Banquo killed, and imagines he sees his ghost. He sends murderers to Macduff’s house, and they slaughter his wife and son, along with the entire household. Lady Macbeth begins to regret having sent him down this path, and soon she kills herself.
Macbeth once again turns to the witches for advice. Now with Hecate in charge, they up their game. They give him a new series of prophecies that are ostensibly warnings, but phrased to make him think he has nothing to worry about. Yet Macbeth is increasingly unstable, decides to go out in battle—to die fighting. He is trying to regain some of his former bravery.
In the beginning of the play, Macbeth is a good man content to be named the next king in due time. He is a good soldier and well liked by the people. He is the obvious choice for King Duncan to name to be his successor. So when the witches predict that Macbeth will become the next king, it is not a surprise for either Macbeth or Banquo. Macbeth seems to put no stock in the witches predictions, but they do plant the seed of evil.
However, King Duncan does not name Macbeth to be the next king, he names his oldest son, Malcolm during a visit to Macbeth's castle. Macbeth's tragic flaw is ambition. Duncan's pronouncement of Malcolm as successor upsets him; however it is his wife's goading, her appeal to his manhood and love for her, that drives him to commit the unthinkable murder. His personality begins to deterioriate at that point.
After the murder of Duncan, Macbeth commits several unthinkable crimes. The second murder Macbeth commits is that of his best friend, Banquo. This murder bothers his conscience as well and the guilt drives him to see Banquo's ghost. The next murder is the slaughter of Macduff's family. Each murderous act is more bloody than the previous and Macbeth becomes more bold and resolute in his ambition to remain king.
After Lady Macbeth's death, Macbeth is alone. Even the witches have abondoned him with their equivocated predictions. Macbeth's death shows the ultimate isolation as he is separated from even himself. Bythe end of the play Macbeth has fallen from a beloved soldier to a man who is deserted even by himself.