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.
The parents have been lawyered up to protect their precious monster as well as the sheriff who had been protecting him over the past 3 years.