He has no qualms killing any of the soldiers. I would think that the problem lies in what the others - the innocent people - may think of him. Surely, news of an entire army company falling to some adventurers wouldn't go unheard of. Any survivors would tell tales of brutality. Or just the evidence of no survivors would be enough. And well, these soldiers may be fathers and sons. They're not bandits. They have each have a family who cares about them.