Devil's advocate..

[Angrboda]

The puzzle you were trying to solve was deciding between the moral rightness of two systems - and to solve that you need some standard or rule to judge moral rightness. I made an assumption regarding what rule would be accepted by both parties - but if that is not acceptable, then state your criteria and I'll make another attempt.

That's just it. I don't have such a standard. I believe that morals are one part evolved psychology and one part cultural conditioning. I see no way to get around that morals are culture specific, and I don't know enough about how evolution influences morals to draw from there.