Daughters of Texas Love Triangle Murder-for-Hire Killer Discuss Dad's Involvement

— -- Haley Shepard remembers the hot summer night in Amarillo, Texas, when her father David Shepard treated his three daughters to a fancy dinner.

She and her sisters recall their typically cash-poor father bragging about his purchase of a new grill and tires for his SUV. It was odd, they said, because their father was “awful” with money, but then seemed to have it “out of the blue.”

“We all asked him, ‘how do you have money?’ and his words were, ‘I did some work for Mike and he paid me early,’” Haley Shepard told ABC News’ “20/20” in an exclusive interview.

“Mike” was her father’s best friend, Dr. Thomas Michael Dixon, who was later accused of hiring David Shepard to kill another Texas doctor. Haley’s recollection of the night of the dinner helped convict Dixon of murder in a second trial after the sisters believe their father fumbled the prosecution’s first trial to protect his friend.

David Shepard’s three daughters, Rachel, Abigail and Haley Shepard, are now speaking out publicly for the first time in an exclusive interview with ABC News “20/20” about their father’s involvement.

“It’s embarrassing,” Haley Shepard said. “[My father] had a chance to make it right … and he completely ruined it.”

Dixon, an Amarillo, Texas-based plastic surgeon, was charged 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 a jealous ex-boyfriend who paid David Shepard to stalk and then kill Sonnier for dating Dixon’s former girlfriend, Richelle Shetina.

At the first trial in 2012, prosecutors said they had substantial evidence that Dixon coerced Shepard into killing Sonnier for him. Police said they found the gun Shepard used at the bottom of a lake behind Dixon’s office and that the gun was registered to Dixon’s brother. They also found surveillance videos from the day after the murder of Dixon and Shepard eating out together at two Amarillo restaurants.

Prosecutors said they had incriminating text messages between Shepard and Dixon, showing texts like, “you have to stay close… watch him in the AM,” “go get em,” and “we need to make it happen ASAP… get r done.”

Finally, the prosecutors said Dixon paid Shepard in the form of a box of Cuban cigars and three silver bars. They said Shepard had sold one bar at an Amarillo pawn shop before the day of the murder and the other two the day after.

They also said that Shepard tried to kill himself after the murder.

The sisters said they were shocked when they heard their father had been arrested for murder.

“Never would’ve ever thought that he would do something like that,” Haley Shepard said. “[But] I do believe it happened the way that the story plays out. Mike asked him to do it -- I believe that’s actually what happened. … He wanted what Mike had, so spending time with Mike made him look better.”

Before Shepard took the stand, prosecutors said Shepard had admitted to police that he murdered Sonnier, and that Dixon had paid him to do it. They said he agreed to testify against Dixon at trial as part of a plea deal to avoid the death penalty -- Shepard pleaded guilty to first-degree murder in 2013 and is currently serving a life sentence.

Shepard was supposed to be the prosecution’s star witness, until he got on the stand, refused to answer questions and the prosecution’s case crumbled.

“[He] ultimately, completely, digressed from what his statement was to law enforcement in the fact that Mike Dixon didn’t have any involvement in this deal," said Lubbock County District Attorney Michael Powell.

When asked why they thought their father changed his story on the witness stand, Abigail Shepard said, “he was trying to help out his buddy. I think that was it.”

“It was cowardly,” Rachel added.

After Shepard’s testimony, his daughters confronted him in jail.

“Daddy, tell the truth,” Rachel is heard telling her father on jailhouse video obtained by ABC News. “You didn’t say anything, you just kept saying that you wouldn’t tell them.”

“No part of this is right, dad,” Abigail continued. “None of it will ever be right, no matter what you do.”

“They think I’m some kind of a monster,” David Shepard told his girls.

“You’ve made yourself a monster, dad,” Abigail said.

Looking back on their confrontation now, Abigail said the whole situation felt “frustrating.”

“I felt like I was scolding a child,” she told “20/20.” “I just wanted the straight truth. I wanted to know why he did it, what his thought was, where he was going with it.”

“He has never given any of us a straight answer, because it’s a different story every time each of us ask,” Haley added.

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.

"Richelle was posting on Facebook all the time... how Doctor Sonnier was ten times the man that Mike Dixon would ever be," Dixon's defense attorney Dan Hurley told "20/20." "And so Mike was just-- his ego was deeply bruised, and he wanted show Richelle that she was wrong."

The defense claimed Dixon never wanted Sonnier dead and Shepard stole the gun from a drawer in Dixon’s home. They also argued that Shepard was paid the three silver bars as a business investment for a company they were starting.

After several days of deliberation, the trial ended in a hung jury. Shepard’s daughters were so disgusted they apologized to the Sonnier family.

“We were there [in the courtroom] more in support of the Sonnier family,” Haley Shepard continued. “Our heart was broken for them.”

“They deserved so much more than that [outcome],” said Rachel Shepard.

Dixon went to trial a second time this past fall. Haley Shepard’s recollection of that night when her father took her and her sisters out to dinner became crucial testimony against Dixon. In retrospect, the sisters said they believe their father suddenly had money because Dixon had recently paid him for the murder.

Dixon was found guilty of two counts of capital murder charges in November 2015. He was sentenced to life in prison without the possibility of parole. Dixon’s attorneys plan to appeal.

“The number one most important thing from this second trial was the unbelievably courageous testimony of Haley Shepard,” Dr. Joseph Sonnier’s son, Dallas Sonnier, told “20/20.”

The Shepard sisters said they don’t talk with their father often, but Rachel Shepard said she would say to him, “we are OK without him.”

“I’m proud of where we are,” Abigail added.

“I think he is where he needs to be,” Haley continued. “I would love for him to be able to see and be proud of where we’re at also.”