SAN JUAN, Puerto Rico—Last week, phones across Puerto Rico began to ring as members of a private WhatsApp group dedicated to helping others in the U.S. territory stared at their screens in disbelief. The businessman they knew as administrator of that chat—someone who organized philanthropic efforts such as a Christmas toy drive for needy children and renovations for an elementary school—had just been arrested. Conor Vincent D’Monte, who went by Johnny Williams in Puerto Rico, was allegedly a leader of a violent gang sought by Canadian authorities on charges including first-degree murder. He had been on the run for more than a decade. “It’s like a Netflix story,” said Antonio Torres, chief deputy U.S. marshal for the district of Puerto Rico. Authorities don’t know exactly when D’Monte, 44, arrived in Puerto Rico, but they believe he had been using the alias “Johnny Williams” for at least several months. The 6-foot, …