Chilling video captures the moment a baby-faced teen casually strolls out of a Harlem building Sunday night — after allegedly fatally stabbing his 16-year-old girlfriend inside.

The eerie footage, which was taken from a nearby deli where the pair fought hours before the slaying, shows suspected killer Zyaire Crumbley, 18, wearing all black as he walks out of the building on Adam Clayton Powell Jr. Boulevard near West 136th Street.

The teen, wearing a balaclava over his head, appears to walk nonchalantly off, although at one point looking behind him, before the footage cuts out. 

Crumbley — who police sources say already has six robbery busts under his belt — was accused by cops Monday of having stabbed girlfriend Saniyah Lawrence in the neck during a fight inside their apartment on Adam Clayton Powell Jr. Boulevard near West 136th Street in Manhattan around 5:40 p.m. Sunday.

Police put out a youthful-looking mugshot of the on-the-lam accused killer in the hopes of trying to nab him.

“He’s out there,” said Lawrence’s distraught maternal grandmother, Denise Jackson, 51, of Crumbley, adding that she knows him and has spoken to him on the phone in the past.

“I just want them to find him,” the grieving grandma said. “His face is on the news. He’s on the run. Just find him and put him behind bars!” 

It’s not clear what sparked the deadly violence. But a first-floor neighbor told The Post on Monday that the teenage couple had moved into the only other apartment on her floor about six months ago, and it seemed there had been nothing but trouble between the pair ever since — with the discord particularly heating up Sunday.

“It always sounded like someone was running and someone was screaming or yelling,” said the woman, a grad student who did not want to give her name “And it’s always been a lot of shouting, a lot of heavy things being thrown around. It’s just always been that way.

“But yesterday, it escalated,” she said. “In fact, it got so bad I had to step out to get something to eat.

“I was telling the detectives I couldn’t really hear what they were saying, only I could hear the noises through the walls, but I couldn’t make out like what they were yelling,” she said.

Juan Carlos Cuevas, 35, a clerk at the Bros Convenience Store 1 where the video was taken, said that about eight hours before the slaying, Crumbley yelled at Lawrence inside the shop, which is about three doors down from the apartment where the deadly stabbing happened.

“They come in here together every day,” Cuevas told The Post on Monday as he looked at photos of the couple and recognized them. “They were here yesterday.

“Usually they make no trouble, just take the money from the ATM machine and buy Arizona iced tea.

“But yesterday at 10 a.m., they come in, and the girl say something, and he yells loud, and I try not to pay attention,” the worker said, adding that he was scared.

Jackson said she can’t begin to figure out why her young granddaughter was killed. 

“Everyone that knows Saniyah would never want to hurt her because she’s not that type of person,” the grandma said. “What was the motive?

“She’s a very lovable person. Why would you do that? Why would you just take her life?”

Authorities say Crumbley is 5-foot-8 and 140 pounds with a dark complexion, medium build and short black hair. Sources said he was wearing a black jacket, black pants and black and green sneakers at the time of the fatal attack.

Crumbley has two prior juvenile arrests and four previous adolescent offender arrests — all for robbery, according to police sources.

His most recent robbery bust was Sept. 11, 2020, the sources said.

Crumbley is also the suspect in two other robberies from 2020 but has not yet been arrested in connection to those crimes, according to the sources.

It does not appear that he has a history of domestic violence with the teen victim, the sources said.

But the troubled young couple’s neighbor said, “I definitely always felt like there was something that was concerning me that was going on in that apartment.

“But I never knew who to advocate for in that apartment because I never saw them.“They had a mat, like a welcome mat, that said ‘grateful,’ and I joked about it because of what was going on in there every night. But it was… terrible!” she said. “I was always being woken up in the middle of the night, yelling, people running, crashing. There’s just very odd behavior all the time.”