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.