The parents should be held liable, that's for sure, when the kids are young. Absolutely. It's the family unit.
You need to know where your children go, who they associate with, and what they do, and you need to start that when they're young when you're a parent. You can't just do it all of sudden when your kid is 18 and you have no idea of how to control them, because by then it's too late.
If they don't have anything to do with their spare time, put them in different groups, in sports, theatre, or whatever. Keep them busy so they're not led astray.