If families of victims or even victims were allowed to choose the punishment there is a strong possibility that they would act out of revenge or bias. Our laws are supposed to be neutral and to afford everyone the right to due process. Giving the victim the final say on punishment would not do this. BTW, are you aware that juries do not decide the punishment? Juries decide the facts and then the judge imposes punishment which is usually specified by some law. In other words the judge does not have the power to punish more than the law allows or to show more mercy than the law allows.