In-party conflict- how do we tell which player's in the right and which in the wrong?

[hamishspence]

And i really hope i dont have to actually explain the difference between slaughtering an animal for food and slaughtering one for pointless cruelty.

It's not about "pointless cruelty" - it's about cruelty that makes a very strong point.

Like I said - I'd rate it as about the same level as whatever they were doing to the ogre while questioning it.

The torture doesnt make it better, although frankly i think im willing to dismiss that as "the players didnt really comprehend that it was torture." but beyond that... why does it become murder just because the Ogre cant act to stop them? Obviously the other ogres werent able to stop the party from killing them either, but i dont see anybody calling that out as murder.

Because that's how warfare in general, works - prisoners have protections. Limited ones, but still some.