Musar על מכות 13:18

In matters involving capital punishment the Torah does not allow a person who has testified as a witness to also function as a judge, as we deduce from Numbers 35,30. The reason is that people should not think that the judge has a personal motive in the outcome of the case. Only the Lord Himself, in Whose power is life and death, can function as both witness and jury. This ruling does not apply in financial matters. All matters involving crimes carrying the death penalty require the utmost care. The tribunal must consist of more than twenty people in order to provide for עדה שופטת ועדה מצלת, "a quorum of ten judges who vote "guilty" and a like quorum of ten judges who vote "innocent." All this in order to give the accused the benefit of any possible arguments in his favor. The wording of the Torah suggests that judgment is not to be proclaimed until it has been proven that all arguments in favour of the defendant have been exhausted Even when the judges themselves have observed the crime committed, a murderer is not to be executed until after he has been properly tried.