If Macbeth kills a king or a famous person, the trial in modern America would probably be highly publicized with daily updates as the trial evolves. Certainly, this would be the case if Macbeth were an American and killed another high-ranking man perhaps in politics or high social circles. The trial would become a media frenzy for better or worse.
Within the trial itself, Macbeth might decide to fight it out to the end and plead not guilty. Remember how stubborn Macbeth became and the lengths he went in order to cover his guilt and protect his future as king. Facing criminal charges, Macbeth might be similarly determined to prove his innocence, as difficult as that might be (with Macduff and Malcolm probably summoned to testify against him). If he was so determined, Macbeth would be willing to testify (and maybe even represent himself). Consider the banquet scene when he sees Banquo's ghost at the table. Imagine a similar scene in the courtroom (perhaps more likely in a Hollywood film about the trial) in which Macbeth refuses to take the stand because Banquo is sitting in the seat. He might even talk to the ghost, denying his crime, as he does in Act Three:
Thou canst not say I did it. Never shake
Thy gory locks at me. (III.iv.49-50)
That being said, Macbeth's lawyer might recognize his client's mental state and attempt to plead insanity. This could work considering that Macbeth's testimony could include the prophecies of witches and seeing ghosts.