Not sure we can blame the parents here. It looks like two people with mental disorders had an altercation. Both mental disorders may have had a parental influence factor, but how much would be hard to judge without more information.
Valid point...but, would you as a parent allow your 12 year old access to a gun or raise him to consider using one on another person?
If you even thought he was mentally challenged or unstable, would you not pay close attention and seek help?