I disagree. In a post-collapse world, ex-military (there will be no more "active duty" since there would be no gov't anymore) would be a problem for the same reason that all the ex-KGB guys who now make up the Russian mafia in the ex-Soviet Union are a problem.
I still think the premise is wrong, but it may be too much of a generality to say that I would trust all ex-military. I understand that the services are facing a gang problem today, and those maggots would be well trained preditors, true enough.