An 18-year-old Watertown man was killed while riding a moped Tuesday morning after authorities said he failed to stop for police in Belmont and ultimately collided with an SUV. The deadly crash occurred at the intersection of Stults Road and Fairview Avenue, Middlesex County District Attorney Marian Ryan and Belmont Interim Police Chief Mark Hurley said in a statement. Belmont police saw the moped driving erratically and on the wrong side of Trapelo Road shortly before 9 a.m.