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Time and again at the trial of Joaquín Guzmán Loera, the Mexican drug lord known as El Chapo, figures from his past braved retaliation from the Sinaloa cartel to testify about his heinous crimes.

There was the professional assassin who described how Mr. Guzmán buried a man alive and burned two others in a bonfire. There was the brother of his closest partner who said he routinely bribed the police, the military and almost every other official in Mexico.

Even one of the kingpin’s mistresses took the stand against him and recalled how he forced her, naked, into a tunnel under a bathtub and dragged her along while fleeing from the Mexican marines.

On Wednesday, one last person from Mr. Guzmán’s past, who prosecutors say was instrumental in his downfall but never testified at the trial, appeared at his sentencing in New York to describe the emotional toll of having been a target of Mr. Guzmán’s assassins.