Then again, she could use the "I'm just a woman" plea and throw her husband under the bus. She could assert that he forced her to comply with the plans to murder Duncan since he is the man of the house (and so he became after her famous "are you a man or a mouse" speech) and she is subjugated to his will as she fulfills her role as dutiful wife. It wasn't her idea, it was his. How could she stand against her husband? She was forced into it. To whom could she possibly go for help? What choice did she have? It is very close to a self-defense tactic, don't you think?
As queen, Lady Macbeth would have access to a skilled attorney, so I will answer your question from the perspective of if I were her lawyer. I would certainly point out that she is mentally incompetent to stand trial to begin with. If she were forced to stand trial, I would argue that she did not administer the killing blows to anyone; furthermore, the most she could be charged with is conspiracy. However, no one would know the content of her conversations with her husband, and he is now dead. Unless there were a servant who came forward and claimed to have witnessed any of those conversations, the most that her house staff witnessed was her bizarre and obsessive hand-washing. I think I would be able to get her off any charges. Her behavior was morally reprehensible; however, other than conspiring, she did not break any laws.
Lady Macbeth was very adept in the art of deceit, wearing a false face to disarm others and hide her real intentions. It was she, of course, who counseled Macbeth to appear as the "innocent flower," while being the "serpent" under it. She was an excellent actress and a skilled liar, as evidenced by her behavior when she welcomed Duncan so warmly, knowing he would be dead in a matter of hours.
Considering these talents and the fact that she was totally unprincipled, Lady Macbeth might just lie very convincingly to a judge. She had nothing to do with Duncan's murder. She loved and revered the King, as many witnesses could confirm. After all, she welcomed Duncan into her home and did all in her power to make him comfortable. When she realized her husband was thinking of killing Duncan to gain the throne, she campaigned vigorously to convince Macbeth to change his mind. Since she was only a weak woman and a wife, she could not betray her husband's confidence or act against him, but she used all her powers of persuasion to prevent Duncan's murder.
She went to bed believing Macbeth had given up thoughts of regicide, and she was horrified to be awakened in the middle of the night to learn Duncan was dead. She believed the grooms had killed him because she couldn't stand to believe anything else. When she learned the truth, Macbeth already had been crowned, and she could not oppose him because of his great power.
She might have actually made this story sound plausible.
Lady Macbeth would not need to testify for reasons of insanity. She could be said to show symptoms of schizophrenia and/or a personality disorder in her duplicity when welcoming Duncan yet planning his murder. Her welcome:
All our service
In every point twice done and then done double
were poor and single business to contend
Against those honours deep and broad wherewith
Your majesty loads our house:(Act 1 Scene 6)
Reflects her duplicity or split personality when contrasted with her inner plans-
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements (Act 1 Scene 5)
She is clearly suffering from delusions as she believes she can be a vessel for evil forces:
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe topful
Of direst cruelty! (Act 1 Scene 5)
Lastly, Lady Macbeth is clearly suffering from Post-Traumatic Stress disorder after the murder of Duncan. She is unable to sleep, has developed an obsessive compulsive disorder
Out, damn'd spot! out, I say!—(Act 5 Scene 1)
and has suicidal tendencies. She would not be fit to stand trial at any point after the murder.
Great question! The actual Lady Macbeth would have plenty to say to a judge, but her strongest arguments would be based on her own claim to the throne. In ancient Scotland, the title of king was not passed down from father to son, but from one branch of the family to the next, and as the only surviving member of her family, the throne should have been her's not Duncan's. Add to that Macbeth's own family claim to the throne and the years of unneccessary conflict and bloodshed caused by Duncan's inability to win in battle and his determination to pass the throne to his own sons, and it's little wonder she was willing to help her husband. Finally, if some historians are accurate, she had even more reason to aid in the murder because Duncan may have had her father and other living family members killed to secure his claim to the throne.
Depending upon when she is arrested, Lady Macbeth's statements may vary. For, once she is ridden with guilt and madness, in Acts IV and V, may not defend herself logically at all. However, if she were arrested shortly after Duncan is discovered dead in Act II, Lady Macbeth would be more assertive in her defense.
Since it has been her plan to lay the bloody daggers beside the drunken guards of Duncan, she would most likely seek to implicate the guards knowing that she has drugged them and they have not seen anything she or Macbeth have done. Also, knowing the penchant of the Elizabethans for the supernatural, she may blame things on evil spirits.
It was the owl that shrieked, the fatal bellman,
Lady Macbeth has heard and noted this sound before she has committed the bloody deed. So, she can claim that she has heard the condemnations of the spirits; no wonder that Duncan was killed on such a night!