I would so blame it on his wife. I would go gung-ho for that defense. I would play the, "he's a victim," card. His wife was overpowering, threatening, and manipulating. She emotionally abused him (when she called him names and so forth) and he suffers from battered spouse syndrome. Battered spouse syndrome made him very suseptible to his wife's influence. Therefore, she's the true murderer. Even though Macbeth took action, her manipulation of him made it such that she might as well have been holding the knife herself. In fact, each time he killed, she was holding the knife, in a metaphoric way. As Macbeth killed each character, he always had his wife's voice in the back of his mind. He was powerless against her and had no choice but to execute her will.
If I were a lawyer for Macbeth, I think the best line of defense would be to try to push the blame off on Lady Macbeth. Macbeth seemed to have decided that he couldn't kill Duncan no matter what the prophecy said, but then his wife started calling him names and questioning his masculinity. While you probably couldn't get the charges completely dropped, you might be able to get them reduced. Depending on when this trial was being held, you could probably use her sleeping confession to the nurse as evidence against her.
Another method might be taking the good 'ole insanity plea. After all, Macbeth was talking to magical witches in the middle of the night and does see apparitions on a pretty consistent basis.
Finally, there is still the third grader playground defense: you can't prove it. Macbeth and Lady Macbeth did a pretty decent job of framing the king's guards in the first place. If you could convince a jury that the subsequent actions were out of extreme guilt for the loss of a king and friend, you'd probably be able to create reasonable doubt!