- Nick Reiner (32, right), son of director icon Rob Reiner (†78) and his wife Michele (†68), was arrested for the murder of his parents. His sister Romy (28, second from right) allegedly gave the police the crucial tip.

BLOODY FUGITIVE: Rob Reiner's Son Checks Into Hotel After Alleged Double Murder, Leaves Grisly Trail!

Los Angeles – In a chilling twist to Hollywood's darkest family tragedy, accused killer Nick Reiner fled his parents' blood-soaked Brentwood mansion and checked into a swanky Santa Monica hotel just hours after the brutal stabbings – leaving behind a shocking "trail of blood" that has cops horrified!

The 32-year-old son of legendary director Rob Reiner and his wife Michele Singer Reiner was arrested Sunday night near a downtown L.A. subway station, booked on murder charges, and is now held without bail. Sources say Nick – who has battled severe drug addiction and mental health issues for years – appeared "tweaked out" when he checked into The Pierside Hotel around 4 a.m. Sunday, mere miles from the family's $13 million estate.

Hotel staff later discovered a nightmare scene: the shower "full of blood," smears on the bed, and windows bizarrely blacked out with bedsheets. "He left a real trail of blood," one insider spilled to TMZ. LAPD detectives swarmed the room Monday to collect evidence.

Nick Reiner is said to have left a bloody mess in a room at "The Pierside Hotel." The hotel is a ten-minute drive from his parents' home.

The horror unfolded after Nick's erratic behavior exploded at comedian Conan O'Brien's upscale Christmas party Saturday night. Witnesses say he acted like a "madman" – "freaking everyone out," arguing loudly with his dad, and harassing guests by asking if they were famous. The Reiners left around midnight, but no one knows if Nick tagged along.

Nick Reiner reportedly had a heated argument with his parents at a Christmas party at the villa of late-night talk show host Conan O'Brien. To the right, a pile of trash is visible, apparently remnants of the party.

By Sunday afternoon, tragedy struck: A massage therapist arrived for the couple's scheduled appointment but got no answer. Suspicious daughter Romy, who lives nearby, rushed over and found her parents stabbed to death. She immediately flagged Nick as "dangerous" to cops.

The Reiners had long fought to save their troubled son, who recently moved back home amid relapses. Nick's demons inspired the 2015 film Being Charlie, co-written with his father about addiction's devastating toll.

No one imagined it would end in such unimaginable bloodshed. As Hollywood mourns the loss of the When Harry Met Sally icon and his devoted wife, insiders whisper the family feared Nick's deteriorating state – a heartbreaking battle against inner demons that turned deadly.