Sept. 20, 2013  -- Trevell Coleman was once a rising voice in the hip-hop scene who seemed destined for stardom.

Then, the rapper from Harlem decided to confess to being the gunman in a cold-case murder, which led him to be sentenced to prison.

Coleman, 39, was more commonly known by his stage name, G. Dep -- short for Ghetto Dependent. So talented, he was signed in his 20s by Sean "Diddy" Combs, who performed with him in his music video, "Let's Get It."

Coleman led a self-proclaimed "gangsta" life and glorified it in rhyme, with raps like "Keep It Gangsta," which includes the lyrics, "keeping America high, and why wouldn't I / Gangstas don't talk, we beat the case and walk."

But that life would suck Coleman back in, and he would pay a great debt for committing a crime, betrayed by the last person anyone would have suspected -- himself.

"That was the only way I could have been absolved," Coleman said when talking about turning himself into police. "Personal sacrifice."

Coleman dropped out of college at age 18 in search of a music career. He funded recording sessions by selling cocaine on the streets of Harlem, N.Y. He dabbled in drugs himself, and for $500, he bought a gun.

In fall 1993, a month before his 19th birthday, Coleman used that gun to mug a stranger, he said.

"He was standing under the scaffolding on Park Avenue, 114th street. ... I was riding my bike," he recalled. "I told him, 'Give me the money.' ... He was kind of, you know, unresponsive."

Then, Coleman said, the stranger started coming towards him.

"The guy grabbed the gun, and I pulled the gun back and that's when I fired," he said. "And the guy winced and I didn't know what happened."

Coleman said he fired three times and then fled on his bike, telling no one what had happened. As he left home the next morning, the police were canvasing the neighborhood and stopped him on the street.

"They said, 'Do you know anything about a shooting that occurred yesterday?' And I said, 'Nah,'" Coleman said. "That made me think he didn't pass away, because they said 'shooting.'"

A week later, Coleman said, he threw the gun into the East River. He stayed quiet about the Harlem shooting for four years and poured himself into his music.

Five years later, his talent caught the attention of Sean Combs, already one of the most powerful men in hip-hop. Coleman said he was offered a $350,000 record deal.

"It was definitely more money that I had ever seen," he said.

Coleman had a daughter with a girlfriend. After that relationship ended, he met Crystal Sutton at a club. The couple was married in 2004 and had twin boys, now 9 years old.

He had fame, fortune and now a family -- and the guilt that had been eating away at him.

"It seemed like [it] just wasn't fair for me to be happy," Coleman said. "I used to curb my happiness, you know. Like, just, 'Hah, wait a minute. I'm smiling too much. I'm laughing too much.'"

"I felt like I couldn't really tell anybody," he added. "I didn't want them to be involved."

Coleman remained haunted by his secret and often wondered what happened to the man he shot.

"I thought about whether or not he had children," Coleman said. "I couldn't believe that I could have this beautiful thing in my life and have done something like that. ... He could have been a father and here I am trying to be a father."

Burdened by what happened, Coleman said his music suffered and Combs dropped him from his Bad Boy label.

"It probably was the drugs," he said. "I was just knee deep in trying to medicate myself and not feel, and numb myself to whatever I was dealing with."

In late 2010, Coleman, then 37, couldn't bear it anymore. He went to the police and told an officer he had shot a man 16 years before.

The police did nothing.

"I guess he just felt like it was so outlandish, so he was like, 'Listen, that was a long time ago. Give me your number and I'll call you,'" Coleman said.

Two weeks later, he went back to the police to confess again.

"I think I was just at a point where enough is enough," Coleman said. "It never went away, me thinking about it, so it was like I had to do something about it."

His memory of the incident was vague. He told police that he remembered the victim as being a light-skinned male standing at the corner of Park Avenue and 114th Street. He didn't know the date. The police soon found a match to a cold case murder of a man named John Henkel, who had been shot on Oct. 19, 1993 at that location. It was then that Coleman learned the man he shot had died.

Suddenly, he was charged with murder. But when asked if he had any second thoughts about turning himself in, Coleman said, "Nah."

"In the back of my mind I knew that 'what if' was always a factor," he said.

Once he resolved to confess his crime, his family was in disbelief.

"I was like, 'No, that's not Trevell,'" Sutton said. "That's not even in his character to do that. He's a kind, loving person."

After the case was made public, Sean Combs talked about it in December 2010 on Sirius XM Radio's "Shade 45."

"G. Dep is one of the nicest artists I ever worked with. ... You could always feel like ... something was troubling his soul, because he was real quiet," Combs said at the time. "But he's the type of guy who wouldn't hurt a fly. ... He did the right thing and manned up to it. But my prayers go out to him. He had a real bad drug problem and that could have caused that situation."

Jim Nelson, the editor-in-chief of GQ magazine, served as the jury foreman in the murder trial of John Henkel and has written about the case.

"You have to concede that that man probably lived in a jail in his house in his head for 18 years," he said. "The hard part about this case is that it involves a guy who didn't really need to come forward, who did. I mean, he needed to come forward for his conscience, but we were wrestling with this as jurors because we kind of thought, 'Why did you come forward?'

"Our mission is very, very specific: Is this guy guilty? And I wrestled with that because I knew. I knew he was. I just didn't almost feel worthy of passing judgment on this guy," Nelson added. "He did kill. I don't go into this with rose-colored glasses. I completely understand that for the people who knew John Henkel, it's a different story, and we have to keep that in mind."

The jury found Coleman guilty of second-degree murder. On May 8, 2012, he was sentenced to 15 years to life in prison. He was 38.

"The case of Trevell Coleman has haunted me," Nelson said. "I think the guy got a bad shake. I think he did something honorable and society punished him for it."

Coleman separated from his wife in 2008, but Sutton said she is still standing by him. She said she is still in doubt her husband shot Henkel, but he is now a different man.

"It's a different Trevell to me, not on drugs. He feels freer and that you can tell by talking to him," she said. "He sounds a lot better, like the Trevell I first met -- caring, loving, trying to do what he can."

To the people who might not understand why Coleman turned himself in, including some of the victim's relatives, Coleman said it doesn't matter because he is at peace.

"I don't feel like I'm cheating life too much anymore," he said "A lot of the burden is lifted, you know what I mean? And that was what I needed to do."