An MS-13 member who goes by the gang name “Animal” received a 40-year prison sentence this week for a conspiracy that included the murder of a 15-year-old boy, the U.S. attorney’s office in Massachusetts said Tuesday. The nickname, which the prosecutors highlighted in the headline of a press release, is particularly telling, coming just a week after President Trump called members of the violent, immigrant-fueled MS-13 gang “animals.” Joel Martinez, 23, admitted he murdered the 15-year-old as part of his initiation as a “homeboy” in Mara Salvatrucha, the gang’s full name. “I stabbed the culero three times,” Mr. Martinez said,...