The kid

The kid was mentally ill... And the parents were in complete denial... "It'll never happen to us"... Sound like familiar thinking? The point this movie is trying to make (well, one among many others) is that the above line of thinking is dangerous and leads to kids going without good mental health treatment. As a result of Mom and Dad minimizing and excusing their son's behavior, as well as giving in to their son's manipulative behavior (caused by his mental illness), because "It'll never happen to us," their son is dead and everyone is to blame.
You could also make a case for the mental hospital being negligent in allowing the child, who was clearly in a dangerous mental state, and putting on a show to manipulate everyone into letting him out, to be released into the custody of his parents, who as I've already pointed out, were deeply in denial about how serious their child's mental health was, and woefully unprepared to keep him safe...
His parents, the hospital, everyone around him wer so worried about the stigma of mental health that they just pretended it wasn't as bad as it really was... And thus did not take enough precautions to prevent him from hurting himself until it was too late.