All it took was a shrug from a stranger to send accused killer Franklin Mesa over the edge, recalled a Bronx man who was allegedly attacked by the “neighborhood menace” last year.

Mesa, 19, was arraigned on a murder charge Saturday for the allegedly unprovoked killing of beloved Norwood resident, Nathaniel Rivers, 35, who was stabbed to death in front of his horrified wife.

When Mesa — who is now being held behind bars on $500,000 cash bail or $1.5 million bond after appearing in Bronx Criminal Court in the murder case — allegedly pounced on an unsuspecting Bryan Diaz in April 2021, was arrested for assault — and quickly released.

“I think he got out the next day,” Diaz, now 20, told The Post.

“I was really mad with the whole situation. When he got arrested, the cops sorta shrugged at me when I asked what else can be done. They were like, “you can’t really do much because the system protects him because he has a mental illness. And I was, like, ‘thanks?’ I was really angry because if he could do this to me, he could do this to someone else or try to kill them.”

 Diaz and his mother, Gabriella Calderon were walking home from his aunt’s house last year when they passed Mesa on the street.

Mesa said hi to Calderon. Since neither one knew Mesa, Diaz said he shrugged at him in confusion. 

Mesa then sucker-punched Diaz, attacking him from behind, the victim recalled.

The two started brawling on the sidewalk, with Diaz managing to deliver a blow to Mesa with his umbrella. Mesa then went inside his building to call for his relatives, Diaz said.

Diaz escaped the assault with just a bloody nose, he said, but was furious with the police response.

Mesa also allegedly sliced another neighbor in the face last year, according to Diaz and another resident, Jay Rodriguez, though law enforcement sources couldn’t confirm the incident.

The deadly attack which finally landed Mesa behind bars happened Thursday at East 205 Street and Decatur Avenue, when authorities and neighbors say the schizophrenic younger man approached Rivers and began exchanging words with him.

Mesa then suddenly plunged a knife into Rivers’ chest while his horrified wife looked on, cops said, mortally wounding the dad of one, who was well-liked in the neighborhood.

Diaz was appalled to hear Mesa allegedly killed Rivers. 

“It’s sad. I think the system really messed up and allowed it to happen,” he said. “They could’ve done more to prevent it.”

Franklin Mesa’s grandmother, Ramona Santos, defended her grandson and claimed he was visiting his mother and siblings on 180th Street when the stabbing occurred. 

“When he came home that day, he didn’t have any blood or bruises on him or anything,” Santos told The Post through a translator Saturday. “He just came home, watched some TV and went to sleep.”

An angry downstairs neighbor heckled Mesa’s grandmother while she spoke with The Post.

“Your boy should f–king burn in hell,” he yelled. “He killed my friend.”

Santos said Mesa has been seeing a doctor to treat his schizophrenia for the past seven years and takes multiple prescriptions every day. She gave The Post a list of the daily medications he was supposed to take, including the antipsychotic Clozapine, which is used to treat schizophrenia. The other medicines were laxatives, probiotics and iron, and he was also instructed to use an inhaler for lung issues.

Surveillance video shows Mesa walking to the scene of the murder, then sprinting away minutes later with a large knife in his hand, prosecutors alleged at his Saturday court appearance. The video also depicts him dropping the knife in a stranger’s yard and “running back to his mother’s home,” the Bronx District Attorney’s office contended.

Mesa lives with his grandmother and aunt, defense attorney Michael Rooney told the court, describing Mesa as “disabled” and noting he is not in school. 

Mesa appeared in court wearing a wrinkled white t-shirt, black shorts, black socks, and white sneakers. He wore a surgical mask and had his head bent down the entire time. His aunt and grandmother were in the back row of the court, but they declined to comment.

Mesa was the type of person people in the neighborhood tend to avoid, said Rodriguez, 34, who has been friends with Rivers for 25 years.

“He assaulted an individual last year — sliced him in the face. So what is he doing on the street again?” Rodriguez said. “We didn’t see him for a month but then he was back home. What’s the point?”

A memorial to Rivers outside his apartment building included a poster board with his photo on it, messages from loved ones, two empty bottles of cognac and at least 50 prayer candles. 

Rodriguez described Rivers as the mayor of the neighborhood. 

“Whenever someone passed by, he’d always say ‘Hi’ and ‘Good morning,’” he said. “If anyone needed help with their grocery bags, he’s the one helping them.”

“I’m distraught,” he continued. “You see the guy one day and then he’s gone the next. I’ll never see his smile again.”