like patrick sherrill, mark barton was never formally diagnosed with a mental disorder. rton's personality fit clinical patterns of a man on the edge. >> the person that i started to work with and the person that he ended up were two different people. he was not the same person at all. >> secretary leigh ann burke saw mark barton's behavior change over time. she started working with him in 1984 in texarkana, texas, where he was employed as a chemist for tlc manufacturing, a company that made cleaners and solvents. >> he was always kidding around and laughing and making jokes in the beginning. like i said, he was like a 6-year-old in the body of a 35-year-old man. i mean, he was just a big dennis the menace at first. >> but then a new mark barton began to emerge. >> he would have temper tantrums when he couldn't get formulas to blend right or he would be trying to make a new product and he would try and try and he would have -- he would throw things and, you know, have these little temper tantrums. >> she noticed barton isolated himself and his first wife debra, whom he ha

mark the end of the u.s. presence in that country. now back to "death and the dentist." >>> barton corbin had been indicted for murdering his girlfriend back in 1990 and his wife in 2004. to mount his defense, he hired top legal talent who argued both deaths were, in fact, suicide and that bart's connection was pure coincidence. >> we've contacted some of the finest forensic people in the country, and all of them say that the evidence is consistent with suicide. >> high-profile attorneys david wolf and bruce harvey were confident they could successfully defend dr. corbin. >> we're going into the courtroom in an attempt to demonstrate that nothing that the state can offer can overcome the presumption that dr. corbin didn't commit either of these offenses and as a result he should be found not guilty. >> the defense team planned to tell the jury that in dolly hearn's case, the family's own independent expert didn't think she was murdered. >> the family hired one of the premier medical examiners, forensic pathologists in the country, and said that there is nothing inconsistent with su