GEN821 Jewish law does not require someone to intervene when the risk to his life is substantial.  Professor Aaron Kirschenbaum explains: “The whole purpose of the obligation ‘You shall not stand by while your neighbor’s blood is shed’ [Leviticus 19:16] is the preservation of life.  If its fulfillment can be accomplished only by the sacrifice of the life [of the world-be rescuer], then its purpose has been undermined and frustrated.  Under such circumstances, the obligation falls away.  But while there is no obligation to intervene, it is still viewed as an act of heroism, and worthy of great praise.  [Example] Abraham risked his life to save his captured nephew Lot, although Abraham was in no way endangered by the forces who had taken Lot prisoner.  TELVOL 2:366