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The Third Murderer says he was sent by Macbeth. Given the timing of the murder, so close to the Banquet, it seems very unlikely that either Macbeth or Lady Macbeth was the Third Murderer, and Macbeth's questions and reactions when the two Murderers report the killing seem odd if he had witnessed it. Most likely the Third Murderer was separately chosen by Macbeth, who by this time is much into mistrust and spying.
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