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.
Given that the cost of administering the punishment would be worth paying to prevent the crime...
You should punish them if they are the kind of person that before they do a crime, accurately consider
And then weigh the gains from the crime against the against the losses of the punishments they should have received or and the gains from the punishments they should have avoided.
OR
If at some point they considered whether to be the kind of person that does the above, and decided not to because that would make them susceptible to punishment of future crimes.
--
I think the above conditions are finally sufficient, but not necessary. Some other kinds of agents are worth to punishing for their future crimes too.
Also, the question I was answering above was not really "when should you punish someone for their future crimes", but "what kinds of people are worth punishing for their future crimes". Maybe that's why the answer is so long.