Musar su Genesi 37:4

ולא יכלו דברו לשלום . Rashi comments that we can learn the praiseworthy character traits of the brothers from the very criticism the Torah directs at them. The brothers were no hypocrites, fawning on Joseph to his face and cursing him behind his back. They were candid and frank. We have a parallel to this frankness in Samuel I 25,3, where Naval is described. Our sages say that he was just as evil as is implied by his very name [hardly a name given to him by his father at birth. Ed.], i.e. he did not bother to conceal his עין הרע, ill-will. The difference between Naval and the brothers was that whereas Naval was offensive to everybody, the brothers refrained from speaking to Joseph altogether. They did not want to be guilty of hypocrisy by speaking to him peacefully, hiding what was in their hearts; they also did not want to be guilty of hateful behavior towards him.

The Torah next exhorts us not to bottle up our hatred against a fellow jew in our heart (19,17), but to tell him openly if we wish to criticize his conduct, i.e. הוכח תוכיח. Proverbs 27,5 provides the rationale, namely that a rebuke reveals loving concern for the person so rebuked. This is a virtue as explained by Rashi on Genesis 37,4 that Joseph's brothers, rather than concealing their dislike of Joseph and flattering him, spoke out about it. One must not contrast this with the statement of our sages who condemned Naval for saying what was in his heart. They even changed the reading of the name of his ancestor from נבל to כלבו so as to emphasize that every evil thing in his heart he expressed with his lips (Samuel 25, Jerusalem Talmud Sanhedrin 2,3). His was a special case; he should not have spoken at all instead of using his power of speech only to denigrate others. Joseph's brothers, on the other hand, had to maintain relations with him. Rather than pretend to like him they chose to say what was on their minds.