Who is really responsible for the death of Duncan?
While the plan to dispatch of Duncan at first is articulated by Macbeth, he soon illustrates his reservations for completing the actions. Without the driving force of Lady Macbeth, the crime may not have actually taken place. Lady Macbeth is the opposite of the maternal, and thus operates in a sense of otherness for women: she is willing to dash out a child's brains while nursing, she calls upon the spirits to turn her milk (maternal nourishment) to gall (poison), and she wishes to stop up her mensus. In this way, Shakespeare has characterized Lady Macbeth as truly malevolent and a deficient female character. She is turning on all things female, as she views femininity as weakness. It is for this reason that she calls her husband womanly, and uses sexual diction to call into question his ability to complete the action--killing Duncan. While she does not actually kill Duncan, she ultimately suffers for her sins through her inability to sleep peacefully. This would tie to the voice in the night that states that Macbeth has murdered sleep and shall sleep no more. Despite the spirits stating Macbeth, it is Lady Macbeth who ultimately suffers from no sleep, which shows the punishment for her crime. In this way, one could argue that it is Lady Macbeth who has killed Duncan.
I'd say, without any ambiguity, that it is Macbeth himself. He lifts up the knife and plunges it into Duncan, and he does it because he wants to be king. There are other factors that might have something to do with his motivation, which I'll go into below. But ultimately, he does the deed. He actually does the murder. If you were putting him on trial, there would be no question of blaming anyone else primarily.
The witches do have something to do with it. They predict that he will be king, which is what leads him to actually think properly about killing Duncan, adn of course what prompts Lady M to insist that he carries out the crime. But, it seems, his "black and deep desire" for the throne was there before the witches prophecies too. They might catalyse him into action. But he does it.
Lady Macbeth also has something to do with it. When Macbeth backs out of doing the deed, she persuades him to do it again (see the enotes link below), and without her, he probably wouldn't have gone ahead with it. She's undoubtedly an accessory to murder, carrying the knives back to help frame the grooms. But he's the murderer.