One of the prohibited forms of behavior is gossip.
The Power of Thought and Speech
לֹא תֵלֵךְ רָכִיל בְּעַמֶּיךָ וגו': (ויקרא יט:טז)
[G‑d instructed Moses to tell the Jewish people,] “You must not go around as a gossipmonger.” Leviticus 19:16

According to the Talmud, gossip “kills” three people: the speaker, the listener, and the object of the gossip. That the speaker and listener deserve to be punished is understandable, but why should the person about whom they are gossiping suffer? The answer is that speaking about another person’s shortcomings does more than just belittle him. Words have the power to bring latent energy into actuality. When we speak about a person’s negative traits, it activates them and reinforces them. As a result, his behavior takes a turn for the worse and he thus incurs punishment.

Conversely, when we speak about the good traits of another person, we reveal and reinforce those traits. We can thus be a positive or negative influence on people; the choice is ours.

It is not only prohibited to speak derogatorily about someone; it is also prohibited to think about them derogatorily. In some ways, thinking negatively about someone is more serious than speaking negatively about them.1