Marshalltown cops say a 25-year-old State Center man shot himself to death after a police standoff.It all started Thursday afternoon when Marshalltown police responded to a reported assault. They found a woman had been beaten, and her attacker, identified as Raymond Mason, had stolen her dad’s van and shotgun. Police caught up with the stolen van early this morning. After a brief chase, Mason drove into a back yard and put the shotgun in his mouth. A police negotiator talked with him for nearly three hours, but at about 5:40 this morning Mason walked from the back yard into the street. Cops shot at Mason with plastic bullets and he fell, but was still able to pull the trigger of the shotgun and shoot himself to death.