A convenience store clerk killed a robber in South Dallas this morning.  Police were called to a Diamond Shamrock on Martin Luther King Boulevard near Harwood around 6 a.m. because of a shooting.  When they got there, the found the robber’s body on the ground.  It started when the man grabbed the cash register and ran.  But the armed clerk chased him down and the two began a tug-of-war over the register.  The clerk says he was struck violently several times and, fearing for his life, he drew his pistol and fired twice.  One of the shots hit the robber and killed him right there.  The cashier is being interviewed by detectives and a police spokesman says their evidence will be turned over to a grand jury to decide if the cashier broke any laws.