Inside Rob, Nick Reiners fight at Conan OBriens Christmas party that left parents terrified before murders

Rob Reiner’s drug-addicted son Nick hacked his beloved director dad and mom Michele to death after a massive blow-up at Conan O’Brien’s Christmas party Saturday — where the nepo baby “freaked out” everyone and left Reiner terrified of what he might do next, pals said.The trio reportedly ended up in a “very loud argument’’ — possibly because Nick was back on drugs and refusing yet another go at treatment after at least 17 stints in rehab since age 15, sources told The Post on Monday.“Nick was freaking everyone out, acting crazy, kept asking people if they were famous,’’ a source told People magazine.The embarrassingly bitter blowout occurred fewer than 24 hours before Nick allegedly fatally slit his parents’ throats in the family’s Los Angeles mansion.“They had had an argument at Conan’s holiday party, and Rob had been telling people that they’re scared for Nick and scared that his mental state was deteriorating,’’ a lifelong family friend who lives near the Reiner home told The Post.“Nick was supposedly off drugs,” but then talk surfaced that he was “not so much off them,’’ the neighbor said, explaining that at the accused killer son’s lowest point over the years, he was “badly addicted to combo of opiates and heroin.”Another resident friendly with the family said, “I know [the parents] wanted him to get help, go to rehab, but he wanted to get help while at home — he did not want to get treatment at a facility.“And I know they have argued about that for years.’’Rob, 78 — a beloved Hollywood legend famed for everything from his acting role as Michael “Meathead” Stivic in the TV classic “All in the Family’’ to directing big-screen cult classics such as “Stand by Me’’ and “The Princess Bride’’ — quickly left the O’Brien get-together with his photographer wife Michele, 68, after the fight, TMZ said.The couple’s daughter, Nick’s sister Romy, a 28-year-old magazine writer and...