For much of his life, Otto Nuss has struggled with mental illness. Back in the 1970s, he committed himself to a psychiatric hospital, and he has long taken medication for depression and anxiety. Two months ago, the 63-year-old school bus driver, who friends say had gone off his medication, burst into the national news when he inexplicably took off with his busload of terrified schoolchildren on a seven-hour odyssey from Pennsylvania to a Washington, D.C., suburb. When Nuss was apprehended, there was a loaded semiautomatic rifle on the bus, and authorities later found 48 weapons and thousands of rounds of...