Rebuke is sometimes necessary, to rebuke wrongdoers. But it is better to speak in calm tones, for a soft tongue breaks strong anger (Mishlei 25:15), whereas rebuke brings out anger in the one giving the rebuke and discord and quarrels from the one being rebuked. After all, not every person knows how to accept rebuke. This is especially true nowadays, when even children hardly accept the rebuke of their parents.
Therefore, when one wishes to reprimand another person, he should do so with soft words. Instead of harsh words let them be sweet. In particular if the reprimand will be in the presence of other people, it must not be a public scolding, for that would be humiliating one’s fellow in public, about which our Sages said that anyone who does so “has no portion in the World to Come” (Avos 3:11). One has to be very careful about this, especially in the synagogue, where there is also the issue of the honor and awe of a synagogue; it is certainly forbidden to scold one fellow there. Even if one sees the fellow making the “golden calf” [in the synagogue,] Heaven forbid, one should not raise his voice to shout at the other person, for by so doing it is only harming the one doing the shouting himself. This can be compared to a person washing his clothing under direct sunlight; the clothing might be becoming whiter but the person is turning blacker. Just like the ashes of the Red Heifer (Parah Adumah) make the impure pure and the pure impure, so too the one who shouts in the synagogue is trying to correct the other fellow but in the meantime he is ruining himself.
Our Sages have already explained (Arachin 16b) regarding the verse, You must admonish your neighbor, and do not sin through him (Vayikra 19:17), that the precondition of the mitzvah of admonishment is that it may not be done in the form of public humiliation; otherwise, the one admonishing will himself be sinning “through him.”
Therefore, if one is trying to stop his fellow from some forbidden activity, he should go to the fellow and speak with him privately and as a calm discussion. It is always appropriate to talk softly, with a low voice, for that is the way of humility and submission. But the voice of fools is with needless shouting that only agitates everyone around them, while the words of the wise which are spoken gently are always enjoyable and pleasing to those who hear them.