Three assailants who cut the throat of an aspiring politician from Britain killed him and tried to rape his female companion early yesterday in the driveway of a Georgetown mansion, police reported, after the couple had returned from a night at the movies. The victim, Alan Senitt, 27, a Jewish activist and volunteer for former Virginia governor Mark R. Warner, died at the scene, police said. The woman, whom police declined to identify, was not injured. Within hours, D.C. police arrested four people in connection with one of the most brutal crimes in years on the affluent streets of Georgetown...