The Torah commands us, Do not stand aside while your fellow’s blood is being shed. We are obligated to spare no effort in preventing our fellow Jew from suffering physical, emotional, or monetary damage.
In a situation where a potential victim is unaware that he is being threatened, one is required to warn him even though this will reflect negatively on the person or persons responsible. Speech of this sort is not merely permissible, it is obligatory. Nevertheless, one must be certain that conditions which render speech constructive have been appropriately met.
Sefer Chofetz Chaim, 10 Kislev, Page 188