The shocking double homicide of Hollywood legend Rob Reiner and his wife, Michele, has stunned the entertainment world, but for those close to the family, it marked the tragic end of a long-running nightmare driven by their son Nick Reiner’s mental illness and addiction.

Nick was thought to be using drugs again, and his parents feared for his mental state over the past few months, a family friend who has known Nick his entire life told The Post.

“Rob had been telling people that they’re scared for Nick and scared that his mental state was deteriorating. These poor people did everything to help this kid,” the friend, who lives a few blocks away from the Reiner home, said.

The tension between Nick and his parents apparently boiled over just 24 hours before Rob and Michele were reportedly found with their throat’s slashed. Rob and Nick got into some sort of fight at a holiday party hosted by Conan O’Brien Saturday night, the source confirmed to The Post.

“Nick was supposedly off drugs,” the source said, but then began hearing he was “not so much off them.”

“Nick has been troubled mentally for his entire life and they have tried everything to try and help him and been unable to,” said the source.

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According to the insider, Rob and Michele spared no expense in their attempt to rehabilitate Nick, sending him to “countless facilities” over the years. Nick’s struggles with substance abuse were severe. The source claims at his lowest point, he was “badly addicted to combo of opiates and heroin.”

Rob and Nick came together professionally for a semi-autobiographical movie “Being Charlie” back in 2015. The movie followed the life of an 18-year-old drug addict who was the son of a movie star running for Congress.

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Nick, 32, is currently in custody in connection with the brutal slayings at the family’s Brentwood estate on Sunday.