NEW YORK (CBSNewYork) — A Brooklyn family and the NYPD are in mourning after police say a retired officer, Thomas Marrinan, was killed Monday night by his friend’s gun.

Sources told CBS2’s John Dias on Tuesday the deadly incident happened while the retired cop was trying to help an elderly man outside a Kensington pizzeria.

Police said Marrinan, 57, and a retired correction officer were attempting to de-escalate a situation with 33-year-old Michael Sotto when it appears the retired NYPD officer was accidentally shot.

READ MORERetired NYPD Officer Thomas Marrinan Killed During Altercation Inside Kensington, Brooklyn Pizza Shop

Dias has learned Sotto has six prior arrests for things like domestic violence, including one related to strangulation. He was charged late Tuesday night with manslaughter, criminally negligent homicide and assault.

Meanwhile, the Marrinan family mourns.

“He’s a great man and he’ll be missed by a lot of people,” nephew Nick White told Dias exclusively.

While fighting back tears, White mustered enough confidence to talk about one of his heroes.

“He’s the reason I became a cop. He helps people without ever thinking about himself,” White said.

Marrinan was a transit officer for more than 20 years. He retired in 2013.

On Tuesday, his bureau took to Twitter to say he left an “indelible mark.”

Everyone around Marrinan will say he never fully gave up the badge.

“Anybody needed help, he was the guy to call,” White said.

That’s what happened on Monday night, around the corner from his own home. Another retired officer called Marrinan and said a known troublemaker was harassing his 86-year-old father in front of a pizzeria at East 3rd Street and Church Avenue in Kensington.

The two men then went to control the situation, but police sources say there was a struggle on the street for his friend’s gun.

“The one round, we believe, was discharged, striking both victims,” an NYPD official said.

The friend was not injured, but cops said the single bullet went through the suspect’s arm and then hit Marrinan in the chest.

Cellphone video shows a desperate attempt to save Marrinan, while his wife holds him in her arms begging him not to die during his last few minutes.

“I don’t know what she is going to do. He was amazing,” one woman said.

Neighbors told CBS2 the couple spent this past Saturday proudly celebrating their son’s high school graduation.

“Tom took pictures of them. We just absolutely love them. Tom and Anna are the greatest, the sweetest people in the whole world,” the woman said.

“He was really just the essence of where we live, so it’s devastating,” another person said.

“It’s sad to lose someone like that,” another person said.

Even some who didn’t know Marranin, like one retired subway conductor who donned an old MTA shirt as he spoke to CBS2’s Alice Gainer, felt compelled to stop by a growing memorial of flowers on East 3rd and Church to light a candle.

“He was a transit cop. I worked in transit 33 years. I just felt bad about the whole situation,” the man said.

Now, the family’s teenage son is forced to enter the next chapter of his life without his father.

“Very close bond. They’re like twins, you know, just father and son, a typical father-and-son bond,” White said.

Marrinan’s son was recently accepted to New York Institute of Technology. There is a GoFundMe page posted for those who want to help the family.

Another note about this officer, you probably noticed his long hair. Well, his family said he’d grow it out and then cut it to donate it to kids with cancer.

CBS2’s Alice Gainer contributed to this report

John Dias