Rob and Michele did the best they could with a problematic son. If Nick chose to use rehabilitation and treatment centers as an opportunity to try to get away with cheating and breaking the rules, that's on Nick.
Children who have the privilege of attending live-in private schools, where the student-teacher ratio is typically 9:1, sometimes act ridiculous because they pretend they are rejecting wealth and privilege, or they are hard-done-by or mistreated neglected little rich kids. Instead of embracing the opportunity, they seek attention as though they suffer due to a good education. For example, they play the diva role rather than be normal and thankful.
Nick is a 32 year old man who cannot blame something that happened at the age of 15 on who he is today. None of us can.
Nick was invited by his parents to attend a holiday party with their friends. He must have been behaving normal prior to the party ... otherwise it's common sense that his parents would have left him at home. Nick chose to be rude and obnoxious to guests after he arrived. His rude behaviour upset guests, and he was asked to leave. He was angry, and argued with his father.
Before the end of the day, Nick took a knife, entered his parents' house, caught them off-guard and slashed their throats.
I don't see any reason to frame this in the context of psychosis, mental illness, drug addiction, or anything other than what it is. Nick resented his father and mother. He was angry. He chose to allow his anger to consume him. He chose murder.
I understand the legal defence argument is that drugs caused him to murder his parents, but I don't think that is what happened. There is no documented research that drug addiction causes someone to become a murderer.