Macbeth is a man who is very easily influenced by others. At the beginning of the play, he is a valiant war hero who loves his country and his king. When he meets the witches, he has never thought of being promoted to Thane of Cawdor or king. They plant these ideas in his head, and he begins to feel ambition creep up.
Macbeth writes a letter to his wife telling her about the prophecies, and she gets greedy. She worries that he is too nice, too “full of the milk of human kindness” to do what must be done—kill the king and take the throne. Macbeth has second thoughts. Duncan is his cousin, and a good king.
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,
Not bear the knife myself. (Act 1, Scene 7)
Macbeth decides he does not want to kill him. Lady Macbeth won’t take no for an answer. She berates him and cajoles him until he finally sees the bloody dagger and kills Duncan.
But screw your courage to the sticking-place,
And we'll not fail. ….
When in swinish sleep(75)
Their drenched natures lie as in a death,
What cannot you and I perform upon
The unguarded Duncan? (Act 1, Scene 7)
Once Macbeth kills Duncan, he begins to change. He becomes suspicious of everyone, and obsessed with the prophecies. He has Banquo murdered, and Macduff’s entire family. He worries about Macduff fleeing to England. All he can think about is keeping the throne, avoiding the prophecy about his children not being king, and killing anyone who stands in his way.
Near the end of the play, Macbeth himself realizes that he has changed.
I have almost forgot the taste of fears:(10)
The time has been, my senses would have cool'd
To hear a night-shriek, and my fell of hair
Would at a dismal treatise rouse and stir
As life were in't: I have supp'd full with horrors (Act 5, Scene 5)
He understands that he is not the man he once was. He has allowed the witches and his wife to manipulate him into a bloody tyrant. It is all his fault. He has destroyed his kingdom, his family, and himself, all because he is so easily manipulated by others.