Question

Why does Macbeth give the order to have Banquo and his son, Fleance, killed?

Answer 1

Macbeth has Banquo murdered because he knows that he is suspicious that Macbeth might have killed Duncan. He also was told by the witches that Banqo's sons would be king. This is the reason why he kills Fleance as well.... 
i hope this helps you :)

Answer 2

Answer:

Macbeth gives the order to have Banquo and his son, Fleance, killed To make sure that Banquo's son won't become king as the witches predicted

Explanation:

When Macbeth received the prophecies from the three witches at the same time as Banquo, he did not think about killing him but as his ambition and greed grew, fed by the words of Lady Macbeth, his fear to see the prophecy about Banquo's son come to reality made him act erratically, therefore, he just wanted to assure his permanence in the crown.