While it certainly seems possible that Macbeth would fight the charges, given how far his ambition and pride have driven him, there is also a possibility that he might just resign himself to his fate in prison, as -- toward the play's end -- he does seem to feel something like resignation and regret. Macbeth realizes that his life doesn't look at all like he might have wanted or imagined it would at this point. He says,
My way of life
Is fall'n into the sere, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have, but in their stead
Curses [...]. (5.3.26-31)
In other words, Macbeth feels that his life, with his increasing age, is starting to wither up like a dying leaf. The things that usually come with old age, such as honor, love, and obedience from loved ones, he does not and will not have. Instead, people curse him. It's a sad picture.
Moreover, Macbeth's wife suffers from a "mind diseased," and she eventually takes her own life, leaving him to deal with his demons alone (5.3.50). When he finds out that she is dead, he gives his famous "Tomorrow and tomorrow and tomorrow" speech where he laments the pointlessness and meaninglessness of life. He declares that life is like a "walking shadow" without substance, and "signifying nothing," like a story told by one who lacks intelligence and sense (5.5.27, 5.5.31). These are not the utterings of a man fulfilled by and pleased with his choices. He does, ultimately, decide to keep fighting Macduff because he has nothing left to lose. However, in a modern-day American trial, sans Macduff, he might simply decide that there is no reason left for him to live, and so he could resign himself to his guilt and go to prison.
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.
It is not good to assassinate the leader of your country, and the US is a country with the death penalty. Given the circumstances of the murder, it is likely Macbeth would face execution.
Given that he is an ambitious man who believes the witches' prophesies about his future greatness, one would imagine that Macbeth would fight the murder charges using all his resources. We can imagine him continuing to try to pin the murder on the servants he killed and framed. After all, they are dead already and cannot defend themselves.
We can imagine Macbeth hiring the most expensive lawyers possible, who would probably welcome such a high profile case. With the right legal team, he just might be able to sway a jury in his favor. On the other hand, he would also undoubtedly consult the witches again about what to do, perhaps once again calling them:
You secret, black, and midnight hags . . .
I conjure you by that which you profess
. . . answer me.
Can we wonder that they might lead him astray?