Practice makes perfect.
The key is that they know they are responsible for their actions, that those actions have consequences, and that the increase in demonstrated responsibility gains privilege while the lack of it costs.
Reaching the higher levels of trust gave our children status among their peers--a status they were not likely to risk by screwing up. We had the advantage of a tight parental network, too, and knew if our kids were behaving any way but the way they should before they even got home, just as our parents did.
If you can't leave them unsupervised then they haven't learned anything. Keep in mind, we're talking about their late teens here, an age when they are able to babysit, drive, etc., but the basics should be instilled by then.
Even with that, they will make their own decisions eventually, and the most constrained children often turn out to be the wildest.
There’s a lot of no good that people get up to that ends way, way before murdering a stranger.
But you know what we’ll be told, it’s not their fault, the gun made them do it.
And these people don’t seem to be very well off, how did these kids get their hands on such high end guns? Aren’t guns expensive?