First, we should establish who we mean by Macbeth. If we are referring to the historical figure who was King of Scotland from 1040 to 1057, then the defense is easy. There is no good evidence that Macbeth was a murderer, a tyrant or a usurper. He seems to have been a good king by the standards of his time and he, or his troops, killed Duncan honorably in battle when Duncan invaded Macbeth's territory. Our defense is that Macbeth was grossly defamed by Shakespeare, Holinshed and numerous other sources; no crimes can be proved against him. We should probably also sue Shakespeare and his sources for libel.
If we are referring only to the character in Shakespeare's play, however, we would have to defend him against his murder of Duncan, and his suborning the murders of Banquo, Lady Macduff, and her children (we do not know exactly how many children, but Macduff refers to "all my pretty chickens," which suggests at least three). Tyranny and usurpation are less precise charges than murder, and will probably not be pressed if the murder charges, particularly the murder of Duncan, can be successfully defended.
We have not specified the legal system under which Macbeth would be tried. If Scottish law applies, there are certain ancient peculiarities of that system which might be helpful to Macbeth, including the possible third verdict of "not proven," between guilty and not guilty. Perhaps the best thing a lawyer could do is what is called "putting the prosecution to proof," which is to say, offering no defense, but simply requiring the prosecution to prove the defendant's guilt beyond a reasonable doubt. Since there are no witnesses, this would actually be quite a difficult task. If one did enter a defense, the best one would be "not guilty by reason of insanity." The defense lawyer could point out that the client is subject to hallucinations, and has claimed to see witches, airborne daggers and ghosts. It is worth noting that a successful plea of insanity will not, of course, lead to acquittal. Macbeth would simply be committed to a mental institution instead of a prison.