Macbeth is responsible for King Duncan’s death because he allowed himself to be manipulated, but the witches and Lady Macbeth share responsibility.
By all accounts, Macbeth was a brave and loyal soldier before hearing the witches’ prophecies. In fact, Lady Macbeth fears that he won’t be able to act on her plan because he is “too full o’ the milk of human kindness.”
Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene 5)
This implies that she does not feel that Macbeth has what it takes to take out Duncan on his own.
Lady Macbeth definitely had ambition and ruthlessness to spare. She wanted Macbeth to be king, and she knew just how to push him. The witches knew that the prophecy would plant the idea in his head, and Lady Macbeth knew that if she made a plan he would follow it.
So while Macbeth did kill Duncan and deserves full blame, we must remember the witches’ role in planting the idea in his head and Lady Macbeth’s, and Lady Macbeth’s role in egging him on, creating the plan, and ensuring he followed it through to the letter. Macbeth deserves blame, but they are certainly accomplices.