New Hampshire court records reveal that for John Madore, who killed a hospital security officer, guns and mental illness intersected in dangerous ways. John Madore was barricaded in his Strafford bedroom, with a loaded 9-millimeter pistol and an assault rifle next to him, warning the police officers on the other side of the door. He had just allegedly choked his sister until she couldn’t breathe and knocked his mother to the floor, according to court documents. “I have firearms,” an affidavit quotes Madore as saying, “and this is not going to end well.” That was Jan. 11, 2016, inside his...