For all we know, they had only one chance, and believed what their kid was saying was the truth when confronted with disturbing images/writings. And honestly, the prosecutor condemning the parents for having the kid return to the classroom instead of an empty home is ridiculous - the parents refusing to let the kid go home and stay at school was probably smart, because it would have given them time to better secure the firearm. When the dad went home and saw the gun was missing from his dresser, it was already too late.

I'm actually against charging the parents with manslaughter given the emerging facts. It's ridiculous to suggest that parents should always be responsible for the actions of their kids. We don't do this for parents who buy cars for their kids, and the kid snags the keys and decides to go for a joyride without his parents, and then kills people.