Donald Trump barely survived an assassination attempt in Butler, Pennsylvania, on July 13—a fatal headshot was missed by millimeters. The would-be assassin, Thomas Matthew Crooks, 20, was perched on an unprotected rooftop less than 200 yards from the rally stage. Secret Service snipers killed him during the attack. What was his motive? We don’t know the official narrative from federal authorities, which is becoming a recurring conclusion of such investigations by the FBI. Yet, a social media clue might point us in the right direction. It's a platform that has a disreputable reputation for being a haven for neo-Nazis and...