This is an opinion good or bad. Found this blog by accident. But they said what many didn't want to consider. They posted too soon after the murders but now with some distance from the killing now is a better time for new, different or unpopular theories/analysis.
BUT still doesn't justify a double murder.
Simply put the author says Reiner was a bad parent. He said Reiner seemed controlling and tried to make things about him. They focus in the movie/autobiography about Nck in which they think Rob Reiner took too much control over the project including filming and the pressers/promotions. They said the film should've been turned over to one his Hollywood friends to help avoid inserting his biases as a director/controlling parent.
Rob Reiner was a bad parent; I'm not surprised his kid killed him - Penelope Trunk Careers Blog
That being said bad parenting is frequently the source of adult issues wether some kind of abusive/controlling environment or enabling. It's could be both here. With all the rehabs Reiner seemed to be just throwing money at 'the problem' hoping Nick would shape up and the problem go away. Then most recently he let his addict son stay in the guest house, not a room in their house. Most addicts aren't given a house and they have to earn their way to any kind of sponsored or subsidized housing.
All that aside. The murderer Nick was an adult able to chose his own path including one that could've taken him away from the family name/Hollywood But he didn't. He chose remain in the same environment that probably created and festered his issues. He also chose to get a knife and go into his parents house and physically kill them.
Is the physical nature of his crime a reflection of bad parenting?