The defendant is charged with First Degree Murder due to a death resulting from a spontaneous fight. First degree murder means that the defendant planned and plotted the death of of the deceased beforehand. A death from such a fight would be at most second degree murder, but more likely Manslaughter from the beginning. The prosecution would greatly weaken their case and credibility by trying him on a First Degree Murder charge.
In the end the hero goes to jail for "Jury Tampering". He says that he did that so that the defendant could get a "Fair Trial". There was no part of the plot of the movie that indicated that the defendant was not getting a fair trial through any prejudice or misdeed.