The gun was locked in a safebox. But on Dec. 17, 2019, Aiden Smith, the son of two divorced Los Angeles police officers, guessed the code: his mother’s birth year, according to court records. The 15-year-old took out his stepfather’s gun and fatally shot himself. It was the second time Aiden, who told doctors he was suicidal and plagued by hallucinations of demons, had found a gun in the La...