לֹא־תֵלֵ֤ךְ רָכִיל֙ בְּעַמֶּ֔יךָ לֹ֥א תַעֲמֹ֖ד עַל־דַּ֣ם רֵעֶ֑ךָ אֲנִ֖י יְהוָֽה׃
Do not go with slander among your people. Do not stand by at your neighbor's blood. I am YHWH.
לא תעמד על דם רעך. לִרְאוֹת בְּמִיתָתוֹ וְאַתָּה יָכוֹל לְהַצִּילוֹ, כְּגוֹן טוֹבֵעַ בַּנָּהָר וְחַיָּה אוֹ לִיסְטִים בָּאִים עָלָיו:
Do not stand by at your neighbor's blood. Witnessing his death, and you are able to rescue him, for instance, if he is drowning in the river, or if a wild beast or robbers are attacking him.
[Sifra Kedoshim; Sanhedrin 73a]
Commentary on the Torah, by Richard Elliott Friedman, p. 381
19:16. Stand by at your neighbor's blood. The meaning of this wording is uncertain. It has usually been understood to mean: to stand by when someone's life is in danger and one could do something to save it (also: to fail to come forward as a witness when one has knowledge concerning a taking of life). This is the opposite of the principle in American law that one does not have an "affirmative duty to rescue." The biblical principle is that one has to help a fellow human being if one is able.
Chizkuni* - Rabbi Hezekiah ben Manoah (c.1220 - c.1260 CE, France)
Talebearing and standing by idly while your fellow Jew’s blood is being shed are part of the same sin. The Torah warns against talebearing as the next step would result in standing by idly while another Jew’s innocent blood is being shed. What started out as being “only” words, is liable to wind up as complicity in murder.
*A compilation of insights culled from the Midrashim, as well as the writings of twenty other Rishonim, including Rashi, Rashbam and Ibn Ezra. However, Chizkuni does not name any of his sources (other than Rashi), in order to encourage objective study, as he felt that one should focus on the message rather than the messenger.
והענין הוא, שאם נשמע אדם מדבר רע בחברו, שלא נלך אליו ונספר לו פלוני מדבר כך וכך, אלא אם כן תהיה כונתנו לסלק הנזיקין ולהשבית ריב. ואמרו זכרונם לברכה בפרוש רכיל רך לזה וקשה לזה (כתובות מו, א).
And the matter is that if a person hears something bad about his fellow, that he should not go to him and tell him so and so is saying such and such, unless his intention is to remove damages or to stop a quarrel. And our sages, may their memory be blessed, said about the meaning of rachil, rach la'zeh ve kashe la'zeh, soft to this one and hard to that one. (Ketubot 46a)