Who was the most responsible for the death of King Duncan?

In act 1, scene 7, Lady Macbeth certainly urges her husband to commit murder, doesn't she?

Was the hope drunkWherein you dressed yourself? Hath it slept since?And wakes it now, to look so green and paleAt what it did so freely? From this timeSuch I account thy love. Art thou afeardTo be the same in thine own act and valorAs thou art in desire? Wouldst thou have thatWhich thou esteem’st the ornament of life,And live a coward in thine own esteem,Letting “I dare not” wait upon “I would."

Worth noting, however, is that Lady Macbeth, for all her urging and pressing, does not want to be the one to murder Duncan herself. She wants the deed finished, and she wants Macbeth to do it for her. So it might be easy to lay blame on her for seemingly pressing Macbeth to commit a murder against his wishes.
However, this assertion neglects one very key fact. Macbeth always has a choice in his actions. To blame the entire murder on Lady Macbeth is to claim that Macbeth is powerless against her. And this isn't true. He could have made different choices. In the end, he wants the prophecies of the witches to come true; perhaps Lady Macbeth is simply a voice for the desires that burn deep within her husband.
Duncan dies because Macbeth kills him. Macbeth longs for greatness and, in the end, is willing to do whatever is necessary to make this dream of his a reality. He is ultimately to blame for Duncan's death.

Legally, both Macbeth and Lady Macbeth are equally responsible for the murder of King Duncan. In legal terms, Macbeth is the direct perpetrator since he stabbed King Duncan until he died. Lady Macbeth is the indirect perpetrator as she coaxed Macbeth into killing King Duncan because she could not carry out the deed. Macbeth is introduced in the play as a brave, ambitious but loyal warrior. From this point of view, the audience is led to believe that he cannot contemplate killing King Duncan. This is justified in the number of times he doubts the merit of the deed with Lady Macbeth manipulating him by bruising his ego. Lady Macbeth, on the other hand, is the brains behind the murder as she plans when and where he must kill King Duncan. She forces Macbeth’s hand by questioning his manhood and belittling him. This shows that, she is the one most responsible for the murder as Macbeth would never had committed such an act had it not been for his wife’s persuasion.

This is an interesting question—and one that requires much thought. Does murder start in the hand, when the act is committed, or does it start in the mind, when the act is first thought of as a necessity? This question needs to be answered before the question of who is most responsible for Duncan's death in Macbeth can be answered.
In the play, we see, of course, that Duncan dies at Macbeth's hand. Macbeth is physically responsible for Duncan's death. But if it hadn't been for the leading of his wife, Lady Macbeth, one can argue that Macbeth never would have killed Duncan. She played a major role in convincing her husband that the deed had to be done. Indeed, the guilt she feels is evident in her compulsive need to "wash" the blood off her hands while she is sleepwalking after the murder has been committed.
However, it is not insignificant that Shakespeare starts this play off with the three witches and their meeting with Macbeth, in which they tell him he will soon be Thane of Cawdor and, eventually, king. They are the ones who first plant the idea in his mind—Duncan must die so he can take over the vacated throne.
Murder usually starts in the mind. Therefore, it can be said that the witches hold the most responsibility for the death of King Duncan.