Why does macbeth give the order to have banquo and his son, fleance, killed?

Macbeth orders his hired assassins to kill Banquo and his son Fleance so that they will not pose any threat to his kingship and also to make sure that the three witches's prediction of Banquo's generation becoming the king of Scotland.  

Explanation:

William Shakspeare's "Macbeth" is the tragic story of Macbeth in his zeal to be the king of Scotland, helped or rather manipulated by his own wife Lady Macbeth. Macbeth and Banquo were friends who had stayed by each other's side when the three witches gave their predictions on their future- Macbeth becoming king of Scotland and Banquo's line/ generation will also sit in the throne. But once he had become king, Macbeth fears his friend as he still remembers the prophecy. If the first part of the prophecy had come true, there is no saying that the second part won't be fulfilled. So, in order to secure his own kingship and future, he makes sure that Banquo or his son won't trouble him or take the throne from him, murdering them through his hired assassins.

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