IMHO they had to have knowledge that their son was suffering from psychological problems. Where they went wrong was not getting medical help for him. I think they were in the denial mode. The results of which we are hearing and reading about since Saturday.
Well, on the one hand, it really is disturbing that the first thing they have to say is that they have NO IDEA why their son did this.
On the other hand, unless the kid was willing to accept medical help, there was absolutely nothing they could do to force him to seek it.
If they knew that he was making death threats and was going berserk on drugs and was worshipping a skull in the back yard, then possibly their one course of action would have been to tell the authorities. Or at least, NOT to pressure the sheriff to let him off.
Better to have the kid arrested than to let him get deeper and deeper into drugs and insanity. Drying out in jail is no fun, but it would have been better for all concerned than what happened. And what they should have feared would happen.
Unless a court orders it, you cannot force a adult child into mental health facilities. I know my 32 year old son is a schizophrenic.