After killing the Macbeth says that he have to kill Banquo and his son.
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Macbeth was told by the Weird Sisters (witches) that Banquo would be father of many kings. Macbeth wants his son, not yet born, to be king and thus carry his line. Macbeth wants to kill Banquo and Fleance to make sure no line of Banquo's becomes king.