LOS ANGELES—A homeless man was charged with murder on Jan. 18 for allegedly punching a 70-year-old nurse in an unprovoked attack at a downtown bus stop, causing her to fall down and strike her head on the pavement. Kerry Bell, 48, was arrested a short time after the Thursday attack on Sandra Shells, who died Sunday at Los Angeles County-USC Medical Center, where she worked as a nurse for nearly four decades. Bell is expected to be arraigned on the murder charge Tuesday afternoon in downtown Los Angeles, according to the District Attorney’s Office. He is accused of attacking Shells for unknown reasons at 5:15 p.m. Thursday at a bus stop at Vignes Street and Cesar Chavez Avenue, near Union Station. Shells was waiting to catch a bus to her job at Los Angeles County-USC Medical Center. The attack renewed calls for increased safety on the Metro transit system, while …