You see the carnage that the empire wrecked upon others and you may conclude that punitive wars are right and just, maybe even wars of conquest. Maybe the Spanish conquest of the "New World" could have been handled better. If it were avoided, what would prevent some other group from replacing the Aztecs and then implementing the same system of horror? A Spanish military force to police the area? Coupled with evangelization by the Church? But otherwise leaving the natives to manage their affairs on their own?