Two people each kill an entire family, mangling their bodies beyond recognition. One did it volitionally, driving a car into a family crowd in an act of premeditated revenge, a textbook case of murder. The second did it unintentionally when a bit of road debris caused a blowout and resulted in a horrible highway accident. The first can and should be prosecuted, the second not.
Intent is everything.
In either case such circumstances should be taken into account at the point when charges are brought or after guilt has been determined and punishment is decided. Not during the actual determination of guilt by the jury.