Michael Sandford stopped calling his mother last May. He was in a foreign country, unemployed and living out of a car, but he had a plan. He had learned how to shoot a gun. He wanted to kill Republican presidential candidate Donald Trump. The autistic 20-year-old British man set out from California at about the same time his mother, Lynne, reported him as missing. In June, Sandford showed up at a Trump rally in Las Vegas. As the candidate spoke before a raucous crowd, the Briton moved close to the stage. He noticed a holstered gun strapped to a policeman's...