Halakhah for Berakhot 57:32

It is from the roots of the commandment [that it is] since the impulse of a man's heart is evil, and sometimes a passion comes to his heart against his fellow; and even if he is an extremely proper man, he is not saved from sometimes sinning. And even if a man stands for [a very long time] upon the ways of righteousness, it is not impossible for him to overturn his thoughts and to become bad. As behold, they, may their memory be blessed, said (Berakhot 29a) that Yochanan served in the high priesthood and at the end [of his long tenure], he turned into a Saducee; [they also said about him that he was at first a true prophet, and at the end, he became a false prophet]. Hence the thing is fit to not rely upon the heart of a man to punish his fellow according to his testimony; and even if the convicted is a total evildoer and the most common of commoners and the one testifying is the greatest sage in Israel. But when those testifying are two fit men, there is an assumption about all of the seed of Israel that they will not agree to testify falsely. And [this type of] an assumption is [powerful] about all things.