Here is why you don't kill your captive insurgents.
It is not squeamish sentimentalism. You turn some of them, and get them back out in the boonies killing their former colleagues.
The Brits did it in Kenya against the Mao-Mao. Used captured, vetted and turned Mao-Mao to go out and kill the Mao-Mao.
The French were brilliant at this in Algeria. they would turn some of their captives. Vet them. Go and get their families to protect them (and as insurance), then send the former guerrillas back into the
bled to hunt FLN insurgents. They would receive help from locals, and promptly turn the helpers in to French authorities (e.g. "Hey, Abdul Kader in Bumville gave us food, ammo and told us French patrols were over the next ridge and that we should watch out.") The French would pay Abdul Kader a visit and tell him, "We know you have been helping the FLN with food, ammo and intel. If we catch you doing it again, we're going to kill you and your entire family. Capice?" Pretty soon, nobody would help the real FLN because they were not sure if they were real or fake FLN. The FLN started brutal sweeps of FLN units, thinking some of their members were feeding intel to the French. Thousands of loyal FLN soldiers were killed by the FLN out of blind suspicion.
See
Alistair Horne,
Savage War of Peace, p. 321-336.
A lot of those "traitors" were really loyal ALN soldiers, that someone merely
suspected of being a turncoat. The ALN was killing itself in Algeria in 1958, and was glad it was doing it.
That is brilliant.