Gilgo Beach serial killer Rex Heuermann’s lawyer revealed the two key reasons why he suddenly flipped and fessed up to viciously killing eight sex workers after years of maintaining his innocence.

The hulking Massapequa Park architect’s defense attorney, Michael J. Brown, claimed his client finally decided to cop to the savage killings that have rocked Long Island for three decades to shield the victims’ families and his own from hearing the grisly details of his cold-blooded slayings at trial.

“He certainly wanted to save the families of the victims the ordeal of going to trial, coupled with saving his family that ordeal — it was definitely a factor,” Brown told reporters Wednesday at Suffolk County court.

Brown added that the twisted killer’s decision also came after two devastating rulings by Judge Timothy Mazzei — allowing all DNA evidence to be used and refusing to sever the charges into separate trials.

He called the evidence “overwhelming.”

Heuermann, 62, admitted Wednesday that he strangled and dismembered eight women, then dumped their bodies along barren stretches of Long Island near Gilgo Beach between 1993 and 2010.

The sadistic married father of two pleaded guilty to first-degree murder for slaughtering Amber Lynn Costello, 27; Megan Waterman, 22; Melissa Barthelemy, 24; Maureen Brainard-Barnes, 25; Valerie Mack, 24; Jessica Taylor, 20; and Sandra Costilla, 28.

He also confessed to killing Karen Vergata, 34, whose 1996 murder had not previously been linked to him.

Part of Heuermann’s plea deal includes working with the FBI’s Behavioral Analysis Unit to assist in other serial killer cases, helping investigators understand what drives his disturbed mind, Brown said.

The guilty plea could also spare his wife and daughter, Asa Ellerup and Victoria Heuermann, from potential liability in pending and expected civil lawsuits filed by the victims’ families.

He will be sentenced on June 17 to three life terms without the chance of parole.