The climax of MacBeth is almost in the exact middle of the play. He got away with the Murder of Duncan, although Dante would put him in one of the lowest places in Hades. The killing of your ruler while offering hospitality is an immeasurable crime.
Having said that, the murder of Banquo is the climax. His son gets away thus leaving room for the witch prophecy to come true. That's when things start going awry. It's all downhill from there.
Answer: Banquo's Murder
