Accuser can not be the judge of a case because then the decision taken out for the case will not be a just decision and will be favored towards the accuser.
Explanation:
Accuser is the person who claims that some one has committed a crime and he accuses that person for a punishment or for compensating the accuser of the loss that he had to bear. The judge is the person who judges a particular case and finalizes a decision.
For any one particular case, a judge of that case can not be the accuser of that case because if the accuser is the judge then he will make the decision in his own favor. In this case there will be injustice done to the accused, so the judge can not be the accuser.
