Lady Macbeth masterminds the plot to murder King Duncan. She carefully devises a foolproof plan in which Macbeth is able to kill King Duncan without being suspected. After reading about the prophecy from the Three Wierd Sisters, she instantly begins to devise how Macbeth can successfully murder the king. When Macbeth begins to have second thoughts about committing regicide, Lady Macbeth insults his manhood and calls him a coward. She then encourages him to follow his ambition and unapologetically take what he wants. Lady Macbeth then assures her husband that the plan will not fail. After Macbeth carries out the murder, Lady Macbeth takes the daggers back into Duncan's chamber and sets them near his deceased chamberlains. She then tells Macbeth to wash the blood off of his hands. Overall, Lady Macbeth is the driving force behind King Duncan's murder. She not only devises the plan but convinces her husband to go through with the murder. She also participates by placing the daggers in position after the crime has been committed.
The lady is solely responsible. Had she not read the letter, or been home when Duncan visited, he would have left without any harm coming to him.
Lady Macbeth comes up with the idea for murdering Duncan while reading the letter her husband has sent her regarding the witches' prophecy. From the moment Macbeth returns home, she is in his ear convincing him of the necessity of the deed. She plans it and puts the plan in action. She even says that had Duncan not resembled her own father, she would have done it herself. After she sounds the bell to send Macbeth to Duncan's chamber and he returns with the daggers, she takes them back to the room and plants them on the guards in Duncan's bedchamber. She is the one who insists on "a little water clears us of the deed" but she does not realize the power of a guilty conscience. She is the first to fall to guilt, sleepwalking and then suicide.