Macbeth’s character degenerates after he kills Duncan because he goes from worry and then regret to killing with reckless abandon.
When we first meet Macbeth, he is a hero. A brave soldier, he acts fearlessly on the battlefield. His bravery endears him to Duncan, who promotes him to Thane of Cawdor.
Unfortunately, the witches’ prophecies are nibbling away at Macbeth. His ambition has caught up with him, and he tells Lady Macbeth about it. She comes up with a plan for killing Duncan, and he agrees to it. Then he has second thoughts.
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)
Macbeth does not want to kill Duncan, because they are cousins and Duncan is basically a good king. The only thing he has done to annoy Macbeth is not name him successor. Yet Macbeth finds a way to talk himself into killing the king.
From there, Macbeth decides he has to kill Banquo, because he knows too much and because his sons are prophesized to be king.