The fiend behind one of the most infamous mass shootings in city history — the “Palm Sunday Massacre” that left eight children and two young moms dead in Brooklyn in 1984 — has been quietly released from an upstate prison.

Christopher Thomas slaughtered the innocents in an East New York apartment on a rainy Palm Sunday — the blood-soaked culmination of a beef with the home’s owner, convicted cocaine dealer Enrique ­Bermudez.

Thomas, now 68, was released nearly three months ago and is ­believed to be living in Queens.

“He doesn’t deserve to be on the street . . . He killed poor, innocent children,’’ retired NYPD Lt. Herbert Hohmann, who led the investigation and testified against Thomas, told The Post on Friday.

Former NYPD Commissioner Ray Kelly said Friday that Thomas’ “heinous crime” warranted the maximum penalty.

The killer was released from the Shawangunk Correctional Facility on Jan. 5 after serving two-thirds of his 50-year maximum sentence and fulfilling other criteria, including good behavior, said a state Department of Corrections spokesman.

“The parole board did not let him out,” the rep said.

“He had five appearances before the parole board and was denied ­every time. He doesn’t need board approval because he served the two-thirds.”

Thomas had been sentenced in 1985 to 25 to 50 years behind bars after being convicted of manslaughter. He dodged a murder rap because it was determined that his heavy cocaine use contributed to his actions.

Investigators were never able to nail down a sole motive for the heinous slayings, whose victims included Bermudez’s six-months-pregnant girlfriend, Virginia Lopez, 24, another woman and children ages 3 to 14.

Thomas regularly bought coke from Bermudez, leading cops to believe he may have been settling an old drug score. Bermudez also told cops Thomas believed Bermudez was having an affair with Thomas’ estranged wife.

Whatever possessed Thomas to riddle the Liberty Avenue apartment with bullets, he left behind a scene that shocked even the most grizzled NYPD veterans.

Detectives walked in to find the television on and many of the tiny victims still sitting upright as though frozen at the moment Thomas executed them at close range with gunshots to the head.

“One child was eating chocolate pudding, sitting on the couch in a suspended state, with the spoon still in her hand, dead,” retired NYPD Detective Bo Dietl told The Post in 2009 ahead of Thomas’ first parole hearing. “There were victims sitting around the living room with fear on their faces after being systematically shot.”

Bermudez wasn’t even home at the time. The only occupant spared was 13-month-old Christina Rivera — dubbed “The Only Survivor” on a Post front page.