Why does Macbeth see Banquo as a threat?this question is taken from act 3 scene 1

The witches predicted that Banquo's descendants would become kings; therefore, Macbeth fears that these descendants may take his place. He says in Act 3, scene 1, lines 70-71: "For Banquo's issue [descendants] have I filed [defiled] my mind;/For them the gracious Duncan have I murdered...." If Banquo lives (or his son Fleance, for that matter), then Macbeth thinks he has killed the king only so that Banquo's children/grandchildren could perhaps wear the crown. Macbeth has no children, no one he can name as heir to the throne. Only Banquo now stands in the way of his happiness and security, Macbeth believes. To eliminate this threat, he thinks he must kill Banquo---and Fleance.