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.

the bundle of the sticks, talks about union and how sometimes people need a different point of view. when the father in this story asked his sons to break the bunch of sticks, they could not, but when they grabbed one stick each, it was easier for them to break, because they worked together.

in hofus the stonecutter shows a simple happy working man, but one day when he delivers a load of stone into the house of a rich man, he wishes to be a rich man as well and so the mountain spirit grants him that wish and many others, until he realizes that even getting all his desires and wishes come true, he just needed to be humble and have genuine reason to live and be a better man each day.