Rob & Michele Reiner Did Not Kick Son Nick Out of Family Home Before Murders

Nick Reiner did NOT get booted from his parents' house the night before Rob and Michele Reiner were murdered in the Brentwood home where they lived with their son ... TMZ has learned.

There has been speculation online Rob kicked Nick out of the house Saturday night after returning from Conan O'Brien's Christmas party.

A source with direct knowledge tells TMZ, Nick left on his own accord. We do not know the circumstances under which Nick decided to leave. TMZ broke the story he and Rob got into a heated argument at Conan's party ... but it’s unclear if the exchange continued back at the Reiner residence.

Our sources confirm … Nick was not mentally sound in the period of time leading up to the murders. A source with direct knowledge tells TMZ … Nick's mental instability went beyond consuming illegal substances. We're told prescription drugs "messed with his head."

Nick checked into a Santa Monica hotel early Sunday morning. Later that afternoon, Rob and Michele were found stabbed to death in their home ... and that evening Nick was arrested near USC campus, near Downtown Los Angeles.

As we told you, Nick was acting strangely in the days leading up to the murders. Employees at a gas station the family frequented said Nick appeared "drained" and "abnormal."

Nick is charged with two counts of first-degree murder, and his arraignment has been pushed back until January. In the meantime, he's being held without bail and is on suicide watch.