The man charged in the brutal cold case murder of his neighbor chillingly kept up a friendly relationship with the young woman’s family as cops botched the investigation for decades, the victim’s kin told The Post Saturday.

Calvin Grant, 52, was arrested last week for the 1992 slaying of Stacy Lynnette Joyner after Brooklyn prosecutors found that no one ever performed DNA tests on swabs taken from her mouth.

But Joyner’s family still has questions: Why didn’t cops test the swabs and find Grant’s alleged semen sooner? Why did officers tell everyone at the crime scene to leave? Why didn’t they question Grant after someone reportedly saw him on the Brownsville roof where the 21-year-old’s lifeless body was found stripped naked with two gunshots to the head?

“He’s a monster,” said Miles Joyner, nephew of the victim. “He just took my aunt and he’s walking around smiling in my face.”

“This case is crazy, it’s like in the movies. The killer has been living right here the whole time. He’s walking around among us, smiling at us,” he said.

Joyner’s mother sobbed as she recalled Grant living nearby scot-free while cops appeared indifferent.

“Nobody cared. It was not a high profile case. Like it was just swept under the rug. He was right there the whole time,” said Jaqueline Joyner Graham, 65.

She remembered that Grant had taken a disturbing interest in Joyner, once telling her son, “I want your sister.”

“I can’t believe they didn’t test those swabs. Everybody suspected him. Everybody said check him. How did they not check? It was just like they didn’t really care” she said.

Prosecutors matched the DNA found in Joyner’s mouth to Grant’s from a sample that authorities had collected from him after a 2000 robbery conviction.

He’s facing 25 years to life.

A few days before he was arrested, Grant asked one of Joyner’s family members for some cash, Graham said.

“Just two days ago he asked my grandson for money,” she said.

Brooklyn DA Eric Gonzalez said new DNA testing methods helped crack the case.

Joyner’s mother said, “They need to put [Grant] under the jail and never let him out.”