Banquo has done nothing wrong. He has been a good friend to Macbeth, never been disloyal nor was he a traitor. But Macbeth was suspicious of him because he was there when one of the prophecies referred to Banquo's heirs being heirs to the throne. Macbeth worries that his good friend will be suspicious of Duncan's murder because he was there when the prophecy said that Macbeth would be king. He wants to avoid a fruitless crown, so he decides to kill Banquo and his son in order to ensure that none of Banquo's issue take the throne after him.