An MS-13 gang member from Long Island posed as a killer in chilling photos — and Monday, he pleaded guilty to being one.

Illegal immigrant Carlos Argueta was 16 when he helped murder fellow gang member Jose Pena three years ago, according to the feds, who released snapshots of the killer wearing rosary beads around his neck — while holding a rifle and pointing a pistol at the camera.

Argueta was charged as an adult in the slaying, with prosecutors using the photos of him loaded up with weapons to help convince the judge of his ruthlessness.

“The defendant was wearing rosary beads, which are traditionally worn by MS-13 members to demonstrate membership, when taken into federal custody on September 20, 2016,” court documents added.

Argueta claimed at the time that the rosary beads “were worn as a religious act,” the papers said.

He was out on bail in another shooting when he helped kill Pena, authorities said.

The killer — an El Salvador native whose nicknames include “Desorden,” which is Spanish for “Disorder” — pleaded guilty to racketeering and firearms charges in federal court in Central Islip, authorities said.

Argueta, who had been the head of the MS-13 subset the Freeport Locos Salvatruchas, faces a maximum of life behind bars when sentenced.

Argueta was one of several members of the ruthless Central American gang charged in Pena’s killing in Brentwood in June 2016.

Prosecutors have said the suspects turned on Pena, 18, after Argueta told them he thought he had become a government snitch — and also might have been gay.

After getting the OK from MS-13 higher-ups back in El Salvador to murder the victim, Argueta assigned “tasks,” such as gathering weapons and a vehicle, to underlings as part of the plot, the feds said.

The killers then lured the unsuspecting Pena into the vehicle and drove him to a wooded area, where they slashed and stabbed him to death.

His skeletal remains weren’t found till October 2016, more than four months later, on the grounds of the Pilgrim State Psychiatric Center.

Argueta also pleaded guilty in the attempted slaying of a rival outside the Brentwood public library the same year.

“With today’s guilty plea, Argueta admitted that while free on bail for shooting a gang rival in broad daylight outside a public library less than six months earlier, he planned and carried out the vicious murder of a fellow gang member on behalf of the MS-13,” United States Attorney Richard Donoghue said in a statement.

FBI Assistant Director-in-Charge William Sweeney Jr. added, “The twisted code the members and leadership of the MS-13 adhere to doesn’t make sense to anyone outside of the gang.”

At least one other gangster involved in Pena’s slaying and the separate attempted murder has already copped a plea and landed a 25-year prison sentence.

Last year, the man, Elmer Alexander Lopez, also known as “Smiley,” admitted to the crimes.

Some of the defendants in the attempted-murder case also were charged in the killings of Brentwood High School students Nisa Mickens, 15, and Kayla Cuevas, 16, in September 2016. The victims were hacked and beaten to death with machetes and baseball bats.

Neither Argueta nor Lopez was charged in the double slay, which grabbed the attention of President Trump and then-Attorney General Jeff Sessions.