The easiest way to defend the protagonist in William Shakespeare's Macbeth is to "play" the insanity card. There are a few ways one could do this.
First, one could use Macbeth's testimony about the witches in order to support an insanity plea. Not many people today would believe that three witches told Macbeth that he would be king and then disappeared into thin air. This defense would support that Macbeth was not in the right mind when he decided to kill Duncan.
Second, one could use the testimony of both Lady Macbeth and the guests at Macbeth's celebratory crowing dinner. Macbeth's actions at the dinner were very peculiar. He did not see an open seat to take at the table based upon the fact that Banquo was sitting at the seat Macbeth was supposed to take. The testimony of the witnesses could speak to the fact that Macbeth was acting "weird" and he had not been himself.
One last defense, in support of Macbeth, would be to use his testimony about killing Duncan's servants. Macbeth could, most certainly, be suffering from Post-Traumatic Stress Disorder (based upon the fact that he saw his king dead and had to kill the king's murderers). Sometimes, PTSD has a strange effect on a person and Macbeth could have begun to lose his mind after the trauma.