With thanks to my father, who taught this to me when I was very little.
But what about situations in which we are clearly meant to judge, one might ask. What about situations in which the characters in Tanakh have flaws and suffer from their improper decisions; surely there is nothing wrong in pointing these out? No, there is indeed nothing wrong with that- so long as one does so respectfully. There is a telling story that appears in the introduction to the Igros Moshe, Volume 8, page 15. A man was stricken with a mysterious illness, and no one knew what it represented. He alone knew, and told it over to R’ Moshe. He explained that he had referred to the daughters of Lot as women who had no care about their sexual exploits, promiscuous women who were so promiscuous that they had no shame, the oldest one even determining to name her child “Moav” meaning mei-av- from my father. That night, he dreamt of two elderly women who came to him and lambasted him for his behavior. “We are the daughters of Lot,” they proclaimed, “and despite what you might think, we did not name our children in this manner because we had no shame. Do you know what we could have done? We could have done as the Christians do, and stated that we had virgin births, and that our children were gods. This would have been extremely easy for us. Instead, we chose to suffer shame and to explain that our children came from our father, and were not the sons of God. That is why I clearly named my son Moav- so that one would not take him to be a kind of Jesus.” For his improper behavior, he was stricken with an illness.
It is clear, therefore, that to pass judgment on any biblical character, to mock them or deride them, is simply not to understand them. What is worse, it reflects on the person who mocks, derides or otherwise disrespects them rather than the characters themselves, as is demonstrated in the Gemara. It is the one who does not express himself properly who is suspected of impure lineage, or who will not assuredly become a leader of Israel. It is incredibly important to take care to address the people and subject matter of what is holy with the reverence and respect that is due them. They may have sinned; they may have had flaws, but that does not give us the right to mock these flaws or to speak to them as we would those who are close to us.