Who is to blame for the killing of King Duncan? Is it the witches, Lady Macbeth, or Macbeth himself?
Macbeth is to blame for the death of King Duncan, however he is influenced by the witches and his wife, Lady Macbeth. In order to understand Macbeth's guilt and his role in the murder, you must consider that he acted out of his own free will.
"The witches in Macbethare present in only four scenes in the play, but Macbeth's fascination with them motivates much of the play's action. When they meet with Banquo and Macbeth, they address Macbeth with three titles: thane of Glamis, thane of Cawdor, and king hereafter."
At no time was he forced magically to kill the King. The witches do not have the power to force Macbeth to murder the King, their role in the murder was limited to the prophecy, the information that they gave Macbeth, motivated him.
In giving Macbeth a glimpse into the future, or so the witches said, all the deeply held ambitions that Macbeth has held in check rose to the surface, along with the stirring of the dark passions that lie within all men. The witches gave Macbeth an incentive to kill the King, but he killed him by his own hand.
Lady Macbeth also has a part in the murder of the King, but she is not more responsible than Macbeth. Lady Macbeth, once she found out what the witches prophecy said, and that part of it had already come true with the king awarding Macbeth the title Thane of Cawdor, makes a passionate speech, pleading with her husband to take the opportunity that will be presenting itself that evening when the King is a guest in their home.
She tells him that this is fate, destiny, providing him with a great chance to kill the king as he sleeps under their roof. It is a singularly unique opportunity, not likely to happen again. She tells her husband that if he doesn't kill the king then he is less of a man, a coward, a wimp. If he doesn't kill the king then he doesn't really love her, please she begs, do this for me, I want to be Queen.
After Macbeth listens to her go on and on, he is still not sure what he wants to do, he has a debate with his own conscience, but it is his own unchecked ambition, he deep desire to possess power, his longing to be king, that spurs him to kill Duncan.
Once he thinks it over, he decides that he wants to kill the king, a man who has rewarded him, who has honored him by visiting his home to celebrate their victory, to celebrate Macbeth's elevation to Thane of Cawdor.
Out of jealousy, out of a desire to push Malcolm and Donalbain out of the way, Macbeth seizes the moment, kills the king, frames the guards and ends up being crowned because the king's two sons flee Scotland in fear for their own lives.
"After murdering Duncan, then framing and murdering Duncan's attendants, Macbeth, disturbed by the witches' prophesy about Banquo's descendants, orders the murder of Banquo and Banquo's son, Fleance."
In my opinion, the fact that Macbeth keeps on murdering, after he kills Duncan is proof that he was solely responsible for the murder. If he had killed the king, and then ruled Scotland as a noble and righteous king, one could suggest that he was influenced by the witches or his wife, but he turns into a murdering tyrant, proof that he wanted to be king more than anything. He killed his friend Banquo, he has Lady Macduff and her family killed, proof that he was drunk with the desire to attain power and to keep it.