Numbers 35:30 "But one witness shall not testify against a person causing him to die"

A judge must be willing to consider all sides of a case and make very sure that an innocent man is not punished.

If a person saw a man commit murder or a theft, do you think that he would be willing to think of ways to declare that man not guilty?

Therefore, this Negative Mitzvah forbids a witness from also serving as a judge. He may only give testimony.

He also cannot add other information to his testimony that might influence the verdict.