BATON ROUGE, La.—Police found Silas Gibbs Jr. early in the morning of March 3, slumped against the seat belt in a red Mustang, with blood streaming from his ears and mouth. He had been shot, allegedly by one of his closest friends.

Mr. Gibbs, 24 years old, is one of hundreds of young black men across the U.S. to die violently in the past six months. Their deaths are overshadowed by tragedies like the massacres at the Aurora, Colo., movie theater and the Sikh temple in Wisconsin, as well as the killing of 17-year-old Trayvon...