Some real life situations where traitors would try to join with an enemy that resulted in the enemy killing the potential recruit are probably what sparked this. Once a traitor, always a traitor, as in having no sense of honor and just jumping around to whoever you feel best with, simply brings up the question when will they betray me? So the logic is to kill all traitors even if they are helping you, because it's not worth the risk. However, paarthurnax has spent an enormous amount of time paying for his debts by teaching the way of the voice to man. Also he says "what is better, to be born good and put no effort into it, or to be born evil and become good through great effort?" him saying that is the biggest reason i spared his life