Here's an edited version of a puzzle from the book "Chuck Klosterman four" by Chuck Klosterman.
It is 1933. Somehow you find yourself in a position where you can effortlessly steal Adolf Hitler's wallet. The theft will not effect his rise to power, the nature of WW2, or the Holocaust. There is no important identification in the wallet, but the act will cost Hitler forty dollars and completely ruin his evening. You don't need the money. The odds that you will be caught committing the crime are negligible. Do you do it?
When should you punish someone for a crime they will commit in the future? Discuss.
And the for the case of punishing past crimes:
I suppose you oppose that too?
There's no point in punishing a (past) undesirable act if nobody who could potentially commit that act is going to become aware of the punishment you inflicted.
Prisons are only a deterrent if people know that they exist.