Star witness changes story, says he decided on his own to kill doctor: Part 7

Shepard took prosecutors by surprise and testified that he killed Sonnier on his own without any involvement from Dixon.
6:22 | 03/09/19

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Transcript for Star witness changes story, says he decided on his own to kill doctor: Part 7
?????? Shocking testimony in the capital murder trial of Thomas Dixon. The prosecution case against Dixon was very strong. You've got a payoff to an alleged him man. You've got a murder weapon. The circumstantial case here seems to have a lot of elements that would favor the prosecution. Everything that the prosecution laid on the table was countered by the defense, including Shepard's access to the gun. See, the defense claimed Shepard stole the gun from Mike Dixon's house, and frank sellers, one of the defense attorneys, showed me where he thought Shepard stole that gun from. They came in here and they're picking out cigars to smoke and Shepard opens this door here, and it's a gun. So that's how Shepard knew a gun was in that drawer. Exactly. The defense team admits, hey, the silver bars changed hands but this wasn't to payoff some murder-for-hire scheme. This was the seed money for some new business collaboration between the two. Did Mike Dixon ever pay David Shepard in silvers and cigars to kill Dr. Joseph Sonnier? Absolutely not. The defense said that the surveilling that David Shepard was doing was really only to get pictures of Dr. Sonnier out with other women so that he could show them to Richelle and say, "This guy is cheating on you." Shepard's reporting back, "Oh, I couldn't find him. And I think Mike became frustrated with it and it's evident from the messages that he was sending. So then "Go get him" and "Whip and spur"? Go get who? Use of lots of Texas panhandle colloquialisms, like, "Get her done, put it on him, go get him." He had a history of using that language with lots of different people. And the reason they say Mike Dixon was so upset was they felt Richelle shetina was trying to throw the relationship with Joseph Sonnier in Mike Dixon's face. In court, the defense pointed to texts between shetina and Dixon when she backed out of that planned vacation. She texts. I'm in love. Enjoy your get away. When Dixon asked who's the lucky guy? She replies new dance partner. And so Mike was just -- his ego was deeply bruised. And he wanted to show Richelle that she was wrong. So from your perspective, it was about Mike Dixon and his ego? Right. The defense grilled you about those text messages. Oh, yeah. They absolutely did. They did. Text messages that I hadn't seen in years. It came out of the blue. It was crazy. Anything in this that you regret from those text messages? To Mike Dixon? No. No. Anything you wish you would've said differently? I suppose. If I went through each individual text message, yeah, maybe. Mike is morally responsible for David Shepard being down but not for the purpose of killing Dr. Sonnier. Mike Dixon never wanted that to happen. Defense attorneys were really eager to make it clear that Shepard decided to become rogue. Even though Dixon wanted him to surveil Dr. Sonnier and Richelle, he just decided on his own to go down and kill this man. But there's one big problem with that theory. Remember, back when Shepard was talking to police, he said Mike Dixon asked him to kill Dr. Joseph Sonnier. I asked him, "You sure you want to do this, Mike?" I said, "If you were -- when you kill somebody, that'll affect you for the rest of your life. You'll have nightmares. It'll bother you." And he goes, "doesn't bother me." When you saw the interview the police did with him, were you thinking, slam dunk, we got him? I don't know if I've every thought something's a slam dunk. If you watch Dave Shepard on that interview, he's calm, he's relaxed, he's telling things that flow very naturally. We certainly believed it because it matched up to physical evidence. Without Shepard, you've got suspicion. You've got circumstantial evidence. But he's the one who can bring it all together. Matt Powell calls David Shepard to the witness stand. And the jury is about to hear -- at least Powell thinks -- that Dixon set him up to go down and paid him to kill Sonnier. Shepard not only had to face the jury, but he had to face the victim's family, and his three daughters, Rachel, Abigail, and Haley. They weren't there to support their father. You were in that courtroom to get closure? We were there more in support of the Sonnier family. Right. Our heart was broken for them. And do you think that Mike Dixon paid your father to kill Dr. Sonnier? Yes. There wasn't a single seat in the courtroom. It was the moment many had been waiting for. Once Dave Shepard got in that chair and started speaking, he made jaws drop in that courtroom. This bombshell drops. Shepard takes the stand and says, "No, I did it at my own choosing. I'm the one who decided to go kill Dr. Sonnier." It just flipped the whole trial on its head. This is the kind of moment that you typically see in a movie where the star witness suddenly decides I'm not going to testify against him. That stuff doesn't typically happen in real courtrooms. It was definitely easy to see why district attorney Matt Powell seemed frustrated. You begin to question him. What happens? Well, he completely digressed from what his statement was to law enforcement in the fact that Mike Dixon didn't have any involvement in this deal. What's going through your mind? Okay. Game on. We impeached him with his prior statements. I mean where he's laying out in great detail what Mike Dixon did. When he got up there on the witness stand and created that circus, I've never hated anyone more than I've hated him in my entire life. The question now is, how much damage did this full-on reversal do the prosecution's case? More importantly what did it do to the jury? When he got on the stand and changed his entire story, it just kind of threw me for a

This transcript has been automatically generated and may not be 100% accurate.

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