Where did the prisoners come from them? And did we really also do the pushing old people into burning buildings and collecting other old people on piles to have their heads bashed in one by one thing? From your two posts the People both considered such things horrible when done to them and there are others speaking out against murdering helpless people. Which is why I had the impression that the prisoners were enemy combatants and such.
The prisoners mostly came from people you captured after who had fled into the night. There are a lot of enemy combatants, but there's a fairly large mix of non-combatant children and women in there as well. (It also depends on how you define combatants, you've captured a lot of men who never raised arms against you.)
The Hundred Bands also didn't push old people into burning buildings. Burning buildings collapsed onto old people huddling inside them. After each raid was over, both you and the Hundred Bands executed the captives you had on hand. When the Hundred Bands did it, it was out of a twisted sense of mercy. When you did it, it was for revenge.
I guess it doesn't work well with our current justice and honor values, but wouldn't simply leaving the elders be have been an option as well? It's not like they are any danger, either of participating in a counter-raid or of mothering new raiders.
If they left them, what would happen? Remember, they expected you to melt away in the night. If they left the elders alone in an abandoned ruin, what was likely to happen? Chances are they starve. They believed instant death was better than that. Even if you went back to pick up the elders (not knowing they were there
and take a risk braving where the enemy might be), they would've been net negative food producers when all of your supplies and structures had been wiped out. Caring for them would've meant starvation. Abandoning the elders would've been the logical, ruthless choice. Killing them was mercy.
They're the ones who approached us about selling their prisoners of war in the first place. They're the ones who thought it was a good idea to sell their prisoners to us without bothering to consider how we might treat those prisoners they wanted to get rid of regardless. Our only fault is being more humane and civilized than they were, a fault I'm glad we had toward their prisoners all the same.
If you adopt the prisoners fully into your tribe, they're actions are your problem. Allowing them to hit the Hundred Bands was provocation and a cassus belli. If you adopt them, they're family and kin are recognized to be responsible for the actions of their family members. They would have a duty to prevent their family members from committing crimes.
They could have broken off contact with us when they realized we weren't treating our prisoners from them in a way that benefited them.
Not really. If they had, they would've lost access to sugar. Not only is that indescribably valuable since it's one of two known methods of preserving food, but refined sugar is addictive. For a culture that only ever experiences otherwise bland food, having access to refined sugar is literally addictive to them.
It also wouldn't have done anything about your adoptees raiding them.
They could have accepted the inherent risks that came with sending a spy into our midst to steal our secrets.
This information is OOC, but the Hundred Bands never made a concentrated effort to try and steal sugar production from you. One of their Heroes got it into their head to try and discover the process so that they could take it back to their tribe. They were trying to cut the Gordian Knot of needing sugar (for Stab and food preservation) and constantly absorbing low level raids from your adoptees.
The tribe never approved such an action. From their perspective, their children go off to deliver an expected shipment to you, only to be viciously murdered. Since you never talked, there was no way to realize this. If the Hundred Bands had heard from you (Trade: Hundred Bands) then they would've apologized and paid reparations for the attempted theft and the subsequent raid. Even after the southern Big Man took over, he would've been exiled or given over to you had the Hundred Bands known you weren't deliberately tweaking their noses and murdering their people. Not any more, though, there's too much bad blood.
This is the same situation as with the Peace Seekers, writ large. Lack of diplomatic actions leads to conflict.