Russell,

A broadly shared moral code with provisions for its propagation and defense is one of Bostrom's examples of a singleton. If altruistic punishment of the type you describe is costly, then evolved hardscrapple replicators won't reduce their expected reproductive fitness by punishing those who abuse the helpless. We can empathize with and help the helpless for the same reason that we can take contraceptives: evolution hasn't yet been able to stop us without outweighing disadvantages.