I don't think anyone would suggest that parents should be held directly responsible for their child's crime BUT it shouldn't be lost that being a parent is one of THE most important roles in society, and bring children up to be law abiding, functional people in society should be taken seriously. Most parents do a decent enough job, but, like in any job, you get some who are bad at it, some who are plain lazy and some who are just not interested.
If your child starts coming to the continual notice of law enforcement, there should be some consequence - mandatory parenting lessons, supervision, specific tasks and so on. Make it optional if you like, but failure to engage carries some sort of consequence if your child goes on to commit a serious crime.
I'm not supporting the idea that a parent can be held responsible for a single, horrific act - that's nuts. But these kids who glorify and carry knives (and perhaps older siblings) and cause anti social problems in their community can and should be put on a different path. The parents have a huge role to play in that and should be helped to do so, with the caveat that if they don't try, then they have to share some responsibility.