Joel "Animal" Martinez, 23, gleefully described the fatal stabbing of a 15-year-old rival gang member in Chelsea, Massachusetts, just across the river from Boston, according to audio and video from federal authorities.

The member of international crime gang MS-13 didn’t know he was talking to a cooperating witness when he detailed Irvin Javier de Paz Castro’s final moments on a street in September 2015. “I stabbed the a**h*** three times, and it was a beautiful thing! Just beautiful,” Martinez said, an FBI transcript shows.

Martinez told the witness, identified in court documents as CW-1, that the victim asked Martinez if he was going to kill him, to which Martinez responded “f*****g yes,” documents show. He then details how de Paz Castro tried to flee after Martinez stabbed him, running a “short distance” before falling. Martinez caught up to him and “finished him off,” court documents said.

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He later spoke of the scream de Paz Castro let out after the slashing, a sound Martinez said he could still hear in his head.

“Hey, man, and you are always going to hear that scream,” CW-1 said.

Martinez wasn’t yet a full member of MS-13 when he killed the teen, federal authorities said. He was “jumped in” — a ceremony in which gang members beat the new member while a leader counts slowly to 13 — after the slaying, authorities said.

That same month, another member, Carlos “Chuchito” Melara lured a 15-year-old boy to a Boston beach, where gang members awaiting his arrival stabbed him to death, authorities said. The death earned Melara a promotion to “homeboy.”

Martinez faces up to life behind bars after pleading guilty in December to racketeering charges in connection with the killing.

But if Martinez’s conversation with the informant sounds like the boasting of a remorseless killer, his attorney’s defense of him paints Martinez as a victim of MS-13’s coercion and influence.

In a bid for a lower sentence, attorney Peter Ettenberg said his client bragged to save his life, the Boston Globe reported.

“In order to survive within MS-13 it is vital to brag and puff oneself up,” Ettenberg wrote in a memo filed Monday in Boston federal court. “The defendant’s ‘braggadocio” is just that...It is counsel’s belief that Joel Martinez was the sacrifice to the threats of MS-13 towards his family that compelled him to join.”

Ettenberg said Martinez’s brother believes his client, An El Salvador native, joined MS-13 so they’d help him get to the U.S., where his mother lives, the newspaper reported.

“That’s a very nice way of saying ‘come to the U.S. and join us and be with your mother, or stay and be killed'," Ettenberg said.

MS-13, a gang birthed in Los Angeles in the 1980s, has been a target of the Trump administration, and was even mentioned in Trump’s State of the Union address in January. The group includes members who are Honduran, Guatemalan, Mexican and from other areas of Central and South America.

Martinez will be sentenced on May 21, the Boston Globe reported.