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

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 :)

"the port is near, the bells i hear, the people all exalting"near and hear