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There is a philosopher who said, and I quote," chaos is the natural order"
Deterrence tactics work, but they are not 100% effective enough to avoid agents from switching their allegiance. That is, like you said, they help to reduce the chances of the likelihood of the emergence of double agents. But they don't solve the problem, utterly.
Some behaviours, like betrayal are part of our lives. No matter how hard we try to do away with them, they will just persist or linger. And, their genesis, can be traced back to ancient civilizations.
There was a philosopher ( I don't remember his name ) who once said," chaos is the natural order ". To mean, some things, co-exist with the presence of human beings, they will endless endure all human efforts and attempts to get rid of them. Because, they are here, in our lives to stay.
Just like, no matter how many prisons and courts we build, there wont come a day when, all crimes will be put to and end in our communities