The responsibility for a death belongs, first, to the one who directly caused the death, and in the case of King Duncan in "Macbeth", then Macbeth is the responsible party. Macbeth stabbed Duncan to death. Macbeth did not act alone, however. His wife, Lady Macbeth, aided and abetted him in this action by getting the guards drunk enough that they fell asleep (or passed out) and were no longer effectively guarding their king. She also served as the one to alert Macbeth to the timing of the murder. She did this when she rang the bell that Macbeth heard at the end of his soliloquy in Act 2, sc. 1. She shares responsibility for the murder and she would, in a legal proceeding, be charged with a crime. The witches would not be held accountable, legally, because all they did was suggest to Macbeth that he might become king. What he did with that suggestion was his own doing.