Confessions from a onetime Mexican drug-cartel hit man

The man, who is not using his real name and spoke to ABC News in shadow, said he is reformed and claimed to feel remorse for those he hurt.
7:25 | 01/11/18

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Transcript for Confessions from a onetime Mexican drug-cartel hit man
The suspects drove up on first avenue, parked just out of eyesight. The victims were out in the middle of the street talking to a subject in a car. Reporter: The Mexican drug cartel hit on an L.A. Street. This man, the assassin. Broad daylight, 9:00, 10:00 in the morning. They had no anticipation of what was going to happen. Reporter: Martin Corona, a hitman for a Mexican cartel in this rare television interview tries to explain what it's like to kill for a living. I've seen the worst that you can see. Reporter: He insisted we speak to him in shadow and martin Corona is not his real name. This was revenge. That's why they carried it out with so much drama and aggression. You didn't get away with anything. Know what I mean? Reporter: Steve Duncan was a probation officer who started working with Dea on cross-border drug cases. One of them tried to get back home into the house, and he was shot as he was walking up the steps towards the front door. Reporter: Revenge for an attempted hit on his cartel bosses, the Arellano brothers. You've got a guy on the ground who's injured, begging not to be killed. Right. Yeah, he's begging for his life. "Don't kill me." When he was begging for his life I did say, "This is from Ramon Arellano." I later talked to the wife who was at the front door -- I wanted him to know why he was dying. He's an assassin, a multiple murderer. Reporter: Steve Duncan had no idea Corona would later become part of his life. Would you say the two of you are now friends? We share a fondness. I have known martin since I wanted to kill him in 1997. Reporter: Corona admits to eight murders. There may be more. Did you end up actually killing people in prison? While I was in prison -- I'd rather not speak about that. Reporter: He was born and raised on the beautiful socal coastline just outside San Diego. I don't believe anybody is just born a natural gangster. Nobody's born a Charles Manson. Reporter: Corona says he ran away from home at 12 and joined a street gang. And that became my family. Are you involved in violence at this point? Uh-huh, absolutely. Reporter: In and out of prison, he almost inevitably fell in with the Mexican mafia and absolutely the cartels. You either accept that life, or you end up on the side of a curb somewhere. The third subject ran down the alley this way -- Their number came up. And unfortunately, it was time for them to go. Reporter: This was his first hit for the so-called Arellano Felix cartel. Inspiration for the movie "Traffic." They controlled the cocaine smuggled through Tijuana, locked in a turf war with the notorious Joaquin El Chapo Guzman. Chapo Guzman was killing their family members, they were killing chapo's family members, it was a bloody battle. The FBI has added a new name to its ten most wanted list, Ramon oarellano Felix. Reporter: Ramon decided who got whacked, as one U.S. Official put it. He tested a gun by killing a pedestrian he happened to pass in his car. He led a death squad for Ramon. How does it change you once you've taken a life? Something Ramon used to sknl say, we're already damned, we're going to hell anyways, so there's no sense having a conscience about it. Reporter: In San Diego he shot a pregnant woman suspected of talking to the cops. Along with her sister, a fashion model. When I shot the two people, I heard her scream in the back. And I looked back there and I seen a little girl back there in the back seat. Reporter: The girl was okay. But a month later, across the border in Tijuana, Corona and his partner killed her father. We hogtied him with his hands and feet behind his back, covered him with a sheet. All of a sudden we hear this gruesome sound. It was just like a watermelon being smashed, you know. They laugh about it. You hear them on the radios talking about the murders that they do. They get off on it. Reporter: A life portrayed in the 2015 movie "Sicario." That's "Hitman" in Spanish. Did the movies get it wrong? Absolutely, there's no loyalty. You're sacrificed at any given moment. Reporter: His friend and mentor was killed on the job. Corona was tired of the violence. The day I walked away from Mexico was the day my daughter was born. I held her in my arms. And I told my wife right then and there, we're done. Reporter: Not so easy. I wasn't in California very long before I got arrested. Reporter: After a weapons conviction, Corona found himself in an interview room opposite the affable but calculating Steve Duncan. He had just gotten back from carping near where I go camping. We talked about fishing. You know, we hit it off. Reporter: Corona was potentially facing murder charges. Life in prison. One of the things that they instill on you is, don't talk, don't talk, don't rat. And I decided, I'm going to do the best for me and for my children. Reporter: Corona ratted. We used him among with many other witnesses to indict the upper echelon of the Tijuana cartel. Reporter: Martin Corona served 13 years for a weapons charge and dealing cocaine. Got out in 2014. He should have got the death penalty. Martin's the poster child for the death penalty. Reporter: Instead, he tours the country with Steve Duncan from time to time, schooling law enforcement on Latino gangs. I want to be able to get my message out there. Reporter: And Corona just wrote a book, "Confessions of a cartel hitman." He lives in a secret location but declines witness protection. When my time comes, my time's going to come, you know? I know how to defend myself. Reporter: Martin Corona claims to be reformed. Claims remorse. For those he hurt. I tried reaching out to them and apologizing. Reporter: That former model he shot point blank in the head survived. Corona reads from a letter he wrote to her. I can never find the words to apologize for my acts. Nor can I ever expect any emotion other than hate from you. But I truly am sorry. Reporter: She says she does not forgive him. Apparently others do. You hear somebody tell you that they didn't blame you for doing the things that you did, it's -- it's a little overwhelming. Still hurts, still bothers me. Reporter: As far as I can tell, martin is one of the worst of the worst. Yeah. Yeah. He was. Not anymore. And let's hope that, you know, he stays that way, you know? Reporter: After our interview, Corona violated his probation, tested positive for cocaine and meth. After a short stint in jail, he's about to be released again. There's mornings I wake up and I wash my face and look in the mirror. And I can still see the monster that I once was. You can never make amends for taking a life. You can never pay that back. Reporter: I'm Nick watt for "Nightline" in Las Vegas.

This transcript has been automatically generated and may not be 100% accurate.

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