I only fight just wars, therefore all my enemies are ontologically evil therefore all actions against them are justified

Explanation: The Romans placed a relatively high value on the idea of Iustum Bellum - just war. Now, this was certainly not always adhered to - and furthermore, was often very far from 'just' as we would recognize it. But the Romans regarded the process of morally justifying their wars as something that made them superior to barbarian polities.