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.