When Hitler took over France and occupied Paris the SS rounded up the collaborators who had helped the Nazi effort and had them shot. They said they could never trust those who had turned against their own people.
I had never heard that but it sure does make sense.
Back when I was a teenager, when there was just simply no reason for me to think rationally at all, I had this flash of insight about men and dating and marriage.
I would never be willing to become involved with a married (or taken) man because if he would cheat on his wife (or girlfriend) with me, he would cheat on me with someone else. A man who would not stay committed to the woman he was with simply could not be trusted.
It's much the same principle.
Machiavelli, from The Prince:
“I must not fail to warn a prince, who by means of secret favours has acquired a new state, that he must well consider the reasons which induced those to favour him who did so; and if it be not a natural affection towards him, but only discontent with their government, then he will only keep them friendly with great trouble and difficulty, for it will be impossible to satisfy them. And weighing well the reasons for this in those examples which can be taken from ancient and modern affairs, we shall find that it is easier for the prince to make friends of those men who were contented under the former government, and are therefore his enemies, than of those who, being discontented with it, were favourable to him and encouraged him to seize it.”