Firstly, it's Manslaughter, not Murder...BIG difference.
Secondly, one can always find a repugnant example of "injustice". They occur every day.
That in no way makes our criminal justice system a joke. It is far from perfect, but it works the vast majority of times. The joke, if there is one, is the oft misused and abused term "criminal justice" system. It is the "legal system", the "court system", the "criminal punishment system", etc. Criminal justice does not really exist at all. For that matter, justice doesn't really exist, not in this life. Hang out a little longer and maybe you'll see the truth in that statement. The best you can hope for in the court system is, perhaps, a little satisfaction, and maybe some retribution, but never "justice".