Hmmmm. Reputation. Money. Or a lil' bit of both.
Issue with sparing all of them is that we gotta house and feed them, and it isn't guaranteed for the lower grade soldiers to even get ransomed out of our custody. It also might, in the eyes of the Ruthenians who suffered the majority of the damage from this raid season, make us seem like we aren't concerned for them. Or care about them. If we just spare the people who, the day before, were kidnapping their women and children, killing the men, and stealing away the wealth of their lands.
I think the middle option is the best here... Send a clear message that we won't tolerate any more raids on our lands, whilst keeping the important people alive for ransoms and leverage. It's just how it was in those days, those without the right relations or connections were just expendable, especially if the army they were captured by doesn't have the means to house all of them prisoner.
[X] Kill all of them except for their commander, Saadet. Hold him for ransom and as a hostage.