The twisted Staten Island teen accused of killing his mother’s boyfriend allegedly gave a full, gory confession – including that he planned to liquify the body in a blender and flush it down the toilet, starting with the brain, law-enforcement sources said.

Damien Hurstel, 19, allegedly calmly admitted to cops at the NYPD’s 120th Precinct that he fatally bludgeoned Anthony Casalaspro, 45, over the head with a meat tenderizer because he wanted to know what it was like to kill someone, sources told The Post.

Hurstel was expected to be arraigned Wednesday morning on murder and criminal possession of a weapon charges, authorities said.

The alleged spare-no-details confession came after cops took Hurstel, who has a long history of mental illness, into custody Monday evening, according to the sources.

Hurstel allegedly told police the brutal slaying unfolded after he argued with his mother during the morning inside their West Brighton home, prompting Casalaspro to tell him to take a walk to calm down, the sources said.

The agitated Hurstel allegedly returned home to find his mother gone, leading to an altercation that ended with him beating Casalaspro to death with a meat tenderizer, the sources said.

He allegedly planned to cut up Casalaspro’s body into pieces that he’d pulverize with a blender, but ran into trouble carrying out his deranged dismemberment plan, according to the sources.

A first attempt to sever Casalaspro’s head with a kitchen knife failed, prompting the sick suspect to grab a hand saw from a shed to finish the grisly job, sources said.

He then allegedly used a hammer in a failed attempt to remove Casalaspro’s brain, but he was interrupted by his younger sister arriving home from school, the sources said.

The blood-drenched Hurstel allegedly chillingly told his sister, “I did something bad, go to your room,” sources detailed.

But the unsettling warning went unheeded, as Hurstel’s sister followed a trail of blood to the bathroom and found Casalaspro’s dismembered body in the tub – with a spoon sticking out of the severed head’s eye socket, the sources said.

The grisly discovery prompted the girl, 16, to call her mother, who rushed to the Cary Avenue house as NYPD cops grappled with the scene, according to sources and neighbors.

“The mom, she just kept saying to her son, ‘Why would you kill him?! I still loved him!’ She was yelling, screaming from outside into the house. Where her son was,” said neighbor Jennifer Diaz, 46.

An eerily calm Hurstel left the home with his hands up when police arrived shortly after 4:20 p.m., sources said.

Diaz said the alleged killer teen’s mom didn’t look at him as cops walked him from the house, hands cuffed behind his back, and a spooky expression on his gore-streaked face.

“He had blood on his face and his Timberland construction boots, but not blood on his clothes,” the stunned neighbor said.