The point is, if you would punish a guilty person for a stabler society, you ought to to the same to an innocent person, for the some benefit.

This ignores the causal relationships. How is punishing the innocent supposed to create a stabler society? Because, in your scenario, it's just this once and no-one will ever know. But it's never just this once, and people (the judge, X, and Y at least) will know. As one might observe from a glance at the news from time to time. All you're doing is saying, "But what if it really was just this once and no-one would ever know?" To which the answer is, "How will you know?" To which the LCPW replies "But what if you did know?", engulfing the objection and Borgifying it into an extra hypothesis of your own.

You might as well jump straight to your desired conclusion and say "But what if it really was Good, not Bad?" and you are no longer talking about anything in reality. Reality itself is the Least Convenient Possible World.