Answer:
Macbeth is a deeply suspicious man. Having murdered his way to the Scottish throne, he's acutely aware that he could just as easily experience the same fate. Deeply insecure in his power, Macbeth has started to become dangerously paranoid; he sees potential traitors everywhere.
The biggest threat comes from Banquo, not because he's a traitor but because he has all the qualities of kingship. Dashing, charismatic, and with a real nobility of character, Banquo has something of Duncan in him. As long as Banquo is around, Macbeth knows that he can never be safe on the Scottish throne.
Macbeth's also deeply in thrall to the witches' prophecies. The Weird Sisters told him that though Banquo will never become king, his descendants will. This makes Macbeth especially nervous about the security of his throne. He now believes he has no choice but to wipe out Banquo and his entire family if he's ever to have some measure of peace.