If one witnessed a crime which subsequently came before a beis din, rabbinical court, and the accused was acquitted of any wrongdoing, it is forbidden to voice criticism of the beis din’s ruling. One must assume that a court comprised of Torah scholars has done its best to ascertain the facts according to the methods prescribed by halachah (i.e. testimony of two valid witnesses), and has ruled in accordance with Jewish law as detailed in Shulchan Aruch. If one was the sole witness to a crime, then his testimony is invalid, no matter how convincing are his arguments. Furthermore, one must come to terms with the reality that humans re limited in their ability to determine and carry out absolute justice, and ultimately, it is in Hashem’s power to correct social injustice.
Certainly one who loses a court case cannot possibly be objective in his evaluation of the beis din’s ruling. The practice of raising questions as to the competence of the beis din, or accusing the judges of bias, is blatant hotzaas shem ra (slander).
Nevertheless, if one suspects that the ruling was the result of an error in the legal process, he could seek the advice of a rabbinic authority and ask that he investigate the matter.
Sefer Chofetz Chaim, 6 Kislev, Page 180