Have you made a mistake today?
Well, so can a judge. Very few legal cases involve physical evidence that could be subjected to DNA testing. For example, it is estimated that, even among murders, only 10% of cases have such evidence. In USA, statistics show that for every eight people executed, one person on death row should never been convicted. Re-evaluate your attitude to capital punishment; killing an innocent person serves no justice. Join the EU in the world marking against the Death Penalty 10 October.
Many external factors can influence the cause of a trial. See for instance how the concept of implicit bias can influence a trial process: http://www.youtube.com/watch?v=6e3ZfKcgQyw