Hitman Never Intended to Kill Target of Doctor's Murder-for-Hire Case

David Shepard, the hitman involved in a twisted Texas doctor's murder-for-hire love triangle case, claims he and the doctor who ordered the hit never intended for the target to die.

"I think the real story is more interesting," Shepard told ABC News' "20/20." "Because it's such a tragedy."

Dr. Thomas Michael Dixon, a Texas-based plastic surgeon, was convicted in the murder of Lubbock pathologist Dr. Joseph Sonnier, who was found shot and stabbed to death in his home in 2012. Prosecutors argued that Dixon was an obsessed ex who paid his friend Shepard to stalk and then kill Sonnier because Dixon was jealous that Sonnier was with Dixon's former girlfriend, Richelle Shetina.

Shepard stabbed and shot Sonnier July 10, 2012, after he arrived home from work and then, he said, drove to Dixon's house to tell him what happened.

"I said, 'Mike, things didn't go like I hoped,'" Shepard told "20/20." "He goes, 'What are you talking about?' And then I broke the news to him, and he's like, 'So you're telling me?' I said, 'Yeah, yeah.' And he goes, 'Well, we'll just have to see what happens.'"

Less than 24 hours later, police came by Dixon's house to question him, but Dixon didn't mention Shepard's name during interrogation.

"My speculation is that he didn't know what to do," Shepard told "20/20." "Plus it looks bad for him ... I was trying to get pictures about this guy that was dating his former girlfriend."

A man named Paul Reynolds, who staying at Shepard's house at the time of the murder, contacted police and said Shepard mentioned that he killed someone in Lubbock.

Reynolds told police he did an online search and found information about Sonnier's death, and thought he might have been Shepard's victim.

Later, when police questioned Shepard, he said Dixon paid him three silver bars and gave him a box of expensive Cuban cigars in return for killing Sonnier. Pawn shop records show that Shepard had cashed in silver bars the day after the murder.

But now, four years later, Shepard is revealing in his own words that he lied to police that night.

"I embellished, told them [police] a story," Shepard said. "Everybody sensationalized this whole story because some love triangle and all this and a murder-for-hire."

Dixon was arrested and charged with two counts of capital murder. Shepard had accepted a plea deal to testify against Dixon in exchange for pleading guilty to first-degree murder charges to avoid the death penalty. Shepard is serving a life sentence at a maximum security prison in west Texas.

Shepard was the prosecution's star witness at Dixon's first trial in October 2014, but he surprised observers when he claimed on the stand that Dixon was not involved in the murder at all and that he acted alone. The defense argued that Dixon only asked Shepard to take photographs of Sonnier out with other women, with the intent to show his ex-girlfriend, Richelle Shetina, but he never wanted Sonnier dead.

They also said the silver bars Dixon gave to Shepard were part of a business investment for a company they were starting together.

Shepard says the defense's argument is the real story. Shepard told "20/20" that Dixon wanted to keep tabs on his ex-girlfriend and asked Shepard to try to take photos of Sonnier with another woman, which Shepard said he agreed to do.

"Seems kind of silly looking back on the thing," Shepard said. "I don't like to really offend anybody. ... I don't want to hurt anybody. ... I sometimes have a hard time saying 'No,' I guess."

But after weeks of stalking Sonnier, Shepard told "20/20," he wasn't able to get any pictures and began having second thoughts.

"It's not normal to go around and try to take pictures of other people," he said. "That's really not a good thing you should be doing."

At trial, the prosecution presented text messages that Dixon sent to Shepard that said, "go get 'em" and "whip and spur," which prosecutors argued showed that Dixon was pushing Shepard to kill Sonnier.

But Shepard told "20/20" that Dixon was just telling him to stay on target to "get a picture of him [Sonnier] with another girl besides Richelle."

Shepard described what he said happened the July night he met Sonnier face to face.

"Things got out of hand. Unintentionally gun fired. I'm leaning in to see if he's okay. I fall in the window, and then I'm running around to see what's going on," he said. "I said, 'Dr. Sonnier, are you alright?' I don't hear anything, go in the garage. He's laying prone. I hear his last breath. I reach down. There's no pulse."

Shepard claims that in a bizarre and misguided attempt to cover up what he said was an accidental shooting, he plunged his knife into Sonnier's lifeless body.

"I did stab him a couple of times. I can't reconcile the number that the prosecution says. Eleven, I think they've said," Shepard said. "I had Dr. Sonnier's phone in my hand, and I considered dialing 911 just to call the authorities and I didn't. And I put it back in, mutilated his bodily remains and tried to cover it and make it up look like something else."

Dixon's first trial ended in a hung jury and the judge declared a mistrial.

A year later, in October 2015, Dixon went to trial a second time for capital murder. The prosecution did not put Shepard on the stand this time, but his own daughter, Haley Shepard, testified that she believed her father lied on the stand in the first trial to protect his friend, Dixon.

Dixon was found guilty on two counts of capital murder Nov. 18, 2015. He was sentenced to life in prison without parole. His defense team said they are going to appeal the verdict.

To this day, Shepard said he still feels "pretty bad" that his three daughters believe he lied on the stand.

"They know me," Shepard said. "They should know when I'm telling the truth and when I'm not, and I was not. I was telling the truth on the stand."

When asked whether he considered himself a killer, Shepard said, "No, absolutely not."

"An idiot, you know? Careless," Shepard said.

"I didn't mean to kill Dr. Sonnier," he added. "The thought of hurting somebody for compensation is foreign to me. ... What kind of person can do that?"

Watch the full story on ABC News' "20/20" TONIGHT at 10 p.m. ET.