A crazed man turned a Sunset Park, Brooklyn, home into a slaughterhouse late Saturday, leaving a 37-year-old mom and her four young children stabbed to death.

Ming Dong Chen, a 25-year-old suspect — preliminarily identified as the children’s mentally troubled older cousin — was arrested after being subdued at the scene, his feet bare and soaked in blood.

Cops seized a machete and scissors from the home as evidence, a law enforcement source said.

The suspect, who neighbors said lived at the home, didn’t flinch as cops led him in handcuffs from the house of horrors, witnesses said — even as one of the children was removed from the house on a stretcher.

The top of the little boy’s yellow pajamas had been cut open at the front by paramedics in their struggle to save him.

He was pronounced dead early Sunday morning at Maimonides Hospital.

“He [the suspect] was bizarrely calm,” said one neighbor, who asked that his name not be used.

“He was completely composed and answering their questions — even as they brought out two black bags with the kids in them,” the neighbor said.

“He was still calmly answering their questions as the stretcher with the bloody child was rolled right past the car,” the neighbor said. “Again, he seemed unfazed.”

The children’s father came home from work too late to save his family, according to neighbors and law-enforcement sources, who stressed early Sunday that their information was still preliminary.

Police identified the victims as 1-year-old William Zhuo, a 5-year-old Kevin Zhuo, a 7-year-old Amy Zhuo, 9-year-old Linda Zhuo and 37-year-old Qiao Zhen Li.

The mother’s sister was present but unharmed, and called 911.

“The father walked in and grabbed [the stabber] and tried to stop him but it was too late,” said one law-enforcement source.

“He’s the father! He’s the father!” one neighbor could be heard screaming to cops as they arrived at the scene.

Desperate paramedics also transported the mother to nearby Lutheran Hospital. She, too, was pronounced dead on arrival.

Not a single stabbing victim survived the murderer’s assault, sources said.

The suspect remained overnight at the 66th Precinct station house.

Additional reporting by Matt McNulty