So this isn't even a case of modern vs setting ideas of right and wrong. The different cultures within that setting have different ideas. Which then asks the question, for a group like the scions what is their value system and how do they apply it. Based off their actions it looks like...

Killing prisoners bad. Interrogation okay.
Killing non-combatants bad. Killing combatants okay. Killing combatants after they yell "time out I want to talk" unthinkable. A sin most foul that can never be committed in a cut scene ever.
Slavery they just turn a blind eye too. Forced combat for prisoners they turn a blind eye too. They will not do these things themselves, but they will not fight against them.

It seems like the Scions just want to save everyone, pass no judgement on any society, except if you summon a primal we will stop you. But not permanently stop you. Just kill the god and give surprised pikachu face when you summon it again 10 minutes later.