A man was fatally stabbed in Brooklyn Tuesday after he asked his assailant to stop taking pictures of his cousin, police and law enforcement sources said.

Severiano “John” Reyes, 36, and the other two victims were standing outside a deli on the corner of Roebling Street and South 9th Street in Williamsburg around 8:30 p.m. when the unidentified attacker began taunting them, according to the NYPD.

A dispute erupted between the foursome when the suspect refused to stop repeatedly taking pictures of Reyes’ cousin.

The argument turned deadly when Reyes approached and asked the suspect to put the camera down, sources said.

That’s when the man fatally plunged a knife into Reyes’ neck, according to cops.

The father of four was rushed to Bellevue Hospital, where he was pronounced dead.

A 45-year-old male was also slashed in the neck and back, while a 27-year-old woman was stabbed in the wrist. Both are expected to survive their injuries.

It is not clear whether the other two victims are the cousin Reyes was reportedly trying to protect.

Their attacker, who has not been identified by police, is still on the lam.

Reyes’ family said the killer was a deranged childhood classmate who attacked as part of revenge over a long-festering grudge.

“This man came up to my brother two weeks ago,” the victim’s grieving sister Yvette Reyes told the Daily News. “He said, ‘You took something from me when we were children.’ This is premeditated murder.”

According to his sister, Reyes and his killer had been in the same special education class more than two decades ago but had not kept in close contact.

“Unfortunately my brother was stabbed in an unprovoked attack by someone we don’t know personally.,” Yvette wrote on a GoFundMe.

“We weren’t prepared for such a traumatic tragedy to happen … my brother was such a good person.”