Labeling people as villains used to be effective deterrence against doing villainous things. When did that change?
I've read multiple times that a large percentage of the crime comes from a small group of people. Jail them, and the overall crime rate drops by that percentage.
Yes. One's past behavior is a strong predictor of future behavior.
> so they should be preemptively jailed even if they didn't do a crime this time?
No, it means that each successive conviction should result in a longer prison sentence.
I am unfamiliar with the reasons to which the varying murder rate is ascribed. If I had to guess, I would guess economics is #1.