"You have to use ex ante probabilities"

This reminded me of something. Why is the outcome of a crime used to sentence criminals, rather than the likeliest outcome? A cop died while driving to a robbery (IIRC), the robber was charged with murder. Does anyone else find that stupid?

In another instance, two kids had a fight, one fainted. The assailant would be chared with assault if the victim survived, or murder otherwise. So whether he committED murder depended on future events.

I bet this has a name. Doctrine of hindsightus perfectus or something. Anyone know?