Banquo appears to Macbeth to be a chief antagonist in Act III--and not without reason, as Banquo has started to suspect Macbeth had a hand in Duncan's murder. For Macbeth, the main cause of worry is the witches's prophecy that Banquo's descendants will gain the Scottish throne. Now that he has the throne, it is no longer enough for Macbeth himself to be king. Given the enormity of the price has had paid, he doesn't feel the throne is worth it if he can't pass it to his own heirs. Macbeth, therefore, has Banquo killed.
Meanwhile, a more dangerous antagonist emerges in Act III. This is Hecate, the head of the witches. She confronts the three witches from Act I, angry that they prophesied to Macbeth without consulting her. She accuses them of being too kind to him, saying he has no interest in or sympathy with the witches at all. Why help him, she says, when he won't return the favor? She is highly antagonist to Macbeth and commands the witches to deceive him and lead him to his downfall. She is a bigger threat to Macbeth than Banquo, but Macbeth doesn't realize this, continuing to trust in the witches's prophecies.