STEPHENVILLE, Texas (AP) — While defense attorneys mount an insanity defense for the former Marine on trial in the shooting deaths of "American Sniper" author Chris Kyle and his friend Chad Littlefield, prosecutors have described Eddie Ray Routh as a troubled drug user who knew right from wrong. Routh's attorneys have said the 27-year-old, who was diagnosed with post-traumatic stress disorder and was taking anti-psychotic medication, was insane when Kyle and Littlefield took him to a shooting range to provide support and camaraderie. Routh, his lawyers say, believed the men planned to kill him. Because Routh was at a shooting range and both Kyle and Littlefield were armed, "it's an easier case than others for the proposition that he believed they were about to kill him," said George Dix, a criminal law professor at the University of Texas, Austin. The Texas criminal code stipulates that in cases involving violent crimes where defendants are found not guilty by reason of insanity, the court can initiate civil proceedings to have them committed. Erath County District Attorney Alan Nash said during the trial's opening statements that the evidence would show that mental illnesses "don't deprive people of the ability to be good citizens, to know right from wrong, to obey the law."