Macbeth should not have killed Duncan, and it destroyed him.
Macbeth was a good soldier before the witches got ideas into his head. He is described as loyal and valiant. He killed the traitor Macdonwald. He is a cousin, friend, and subject to Duncan and he seems to respect him.
Lady Macbeth is thrilled to learn that the witches said that Macbeth would be king. She too says that Macbeth, though not without ambition, does not really have it in him to hurt anyone.
Yet do I fear thy nature;
It is too full o’ the milk of human kindness(15)
To catch the nearest way. (Act 1, Scene 5)
The fact that even Lady Macbeth thinks that Macbeth is a sweetheart shows that he was not a murderous fiend at the beginning of the play.
Of course, it does not take Macbeth long to change. He gets the ambition bug, and decides that being king is his rightful place. He teeters on the edge, and finally the floating dagger (and his nagging wife) convince him to take action. Once he takes action he is so paranoid that he turns into a killing machine, killing anyone who stands in his way.
Our choices make us who we are. When we make some choices, they change our lives forever. This was the case with Macbeth. He made the choice to kill Duncan, and his life went on a completely different path. He became a different person, and he suffered because he did not like the person he had become.