Transcript for Killer describes what happened during doctor's final moments: Part 5
Let me walk just a second. That's fine. When you have a suspect literally provide you in a very straightforward manner -- He was saying, "Well, I'd just like to cause him some problems." -- A play by play, frame by frame -- He said, "We have to follow him." -- Of a crime that's occurred from the very beginning. How could this get any better? David Shepard is singing like a canary, implicates Dr. Dixon in the plot, said he had to get even with this guy for basically taking his girl away. He pretty much wanted to break up Dr. Sonnier and Richelle. Shepard tells police that while they're drinking the whiskey and sharing the cigars, they are now in the midst of developing a plot to actually commit murder, the murder of Dr. Sonnier. He wanted him hurt and then it gradually developed into having him killed. 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." They plan, and they plan, and for months, and months, and months. As this plan developed between Mike and me, the plan was laid out. The best way to carry forth his objective was to stake out in his backyard, wait for him to come home. Let's talk about the actual day that this this happened. Yes, sir. I believe it was a Tuesday. Tuesday, July 10th. Today in Lubbock, cooler temps that will dip into the upper 70s and lower 80s. Shepard makes the drive from Amarillo to Lubbock, Texas, about a two and a half hour drive, and gets to Joseph Sonnier's house at around 4:00. Hugged the fence down the alley and entered his backyard. Pulled the chair from his outdoor dining table. He has a curbed, brick fence by his driveway. On the west side, there's a tree by that fence. And I'd sit underneath there. And were you still having contact with Michael Dixon during that time? Mike was encouraging, "Keep the faith. Be patient, be patient, he'll be there eventually." For Dave Shepard, the only person he can trust and go to with any issue in this case is Dr. Dixon. In Dr. Dixon's mind, Shepard's the perfect guy to carry out this mission because Dixon's like the boss. He's the prominent physician and Shepard is essentially a loser and the Patsy's ready to do whatever he's told. And he's waiting for two and a half hours, and the guy falls asleep. He's awoken by the sound of someone knocking on the window from the inside and it's none other than Dr. Joseph Sonnier. I wave at Dr. Sonnier and get up. I walk to the window. He's lowered the window about this far from the top down. What happens at that point? I point the weapon with the bottle at him, discharge it several times. After three or four discharges, he's backing away. He trips on his feet and falls to the ground. Shepard admitted that that he used a Gatorade bottle to try and muffle the sound of the gunshot and that he had learned that technique by watching a movie that starred Steven seagal. So I push in the window and crawl in. Go around the corner. As I turn around down the hallway, right into the garage, he's laying on the floor, not moving, not breathing, nothing. I check his pulse in his neck. He's dead. And what happens? I pulled the knife out of my bag and I stuck him in the vital organs, two or three, four times. I find that bizarre. It's like he's so determined to make this right for whatever his relationship might be with Dixon, that he wants to complete the task. According to Shepard, after he kills Dr. Sonnier, he then drives back to Amarillo. So did you recount for Michael Dixon the events? Yes, sir. He was happy about it. He was fine with it. He said "It sounds like you got away clean." And the very next day he's caught on camera having a meal with Mike Dixon. Shepard told police that Dixon became extremely rattled by the fact that detectives had come over to talk to him about the death of Dr. Sonnier. He was nervous. He said I had to get out of town. Dixon recommended that David Shepard get the heck out of dodge, that he leave the area which would in Dr. Dixon's mind keep him from being investigated by Lubbock police. But David Shepard seems to be overcome by what has happened and maybe his role in this. He attempts suicide, cuts his wrists. If you can imagine the disgust that you can feel with yourself after being involved with what I did. Then I pulled out a filet knife and tried to cut myself. It's bleeding like a stuck pig. The first person he calls after that, Mike Dixon. Dixon and Shepard are caught on surveillance video again heading into Mike Dixon's office where Mike Dixon stitches him up. Dave Shepard has basically given you a roadmap as to what happened, and you see how he directly ties Dixon into planning this murder. You now clearly have built a case against Dr. Dixon. Two suspects are in custody in connection with the murder of a Lubbock physician. One doctor's dead and two are under arrest. Police arrested Dr. Thomas Dixon and David Shephard both from Amarillo. Dr. Sonnier was found shot and stabbed in his living room. I cannot believe that they did this. It makes me so angry. A lowlife scum and a piece of Doctor. Like it or not, Mike Dixon is an evil man. And David Shepard is an evil man. It seemed like a slam dunk for prosecutors. They had the man who admitted to killing Dr. Joseph Sonnier as a witness. As they say in west Texas, everybody thought that this case was done and dusted. But is the case against Mike Dixon too good to be true? You got to remember, the prosecution's star witness is Dave Shepard. I mean this is a guy with a history as a conman and a swindler. Shepard is known for telling all kinds of tall tales. Dave Shepard is a con artist of the ultimate magnitude. The prosecution painted Mike Dixon as this puppeteer. If he was that manipulative, this was the dumbest planned murder in the history of the United States. To choose Dave Shepard? What he ends up doing in the courtroom just turns this case upside down. He made jaws drop in that courtroom. This is the kind of moment that you typically see in a movie. The case is going to blow up in the prosecutor's face -- and a convicted man walks free
This transcript has been automatically generated and may not be 100% accurate.