Woman Allegedly Tried to Pin Husband's Murder on Own Son

A man who testified against his mother at her trial for the murder of her husband Gregg Williams says she suggested his brother might have been the trigger-man.

"She said, 'Do you think your brother could have done it?' She asked me if her own son could have killed Gregg," Andrew O'Brien, 26, told ABC News' "20/20."

"She had to plant a reasonable doubt that someone else could have been there that night, someone else could have squeezed that trigger, [by pinning it on me]," O'Brien's older brother, Lee O'Brien, 28, told "20/20."

Andrew and Lee O'Brien's mother, Michele Williams, 42, from Keller, Texas, is serving a 60-year prison sentence for the murder of her husband Gregg Williams. The brothers are Michele Williams' sons from previous relationships.

Gregg Williams was found dead from a single gunshot wound to his head at his home on Oct. 13, 2011. In an interview with police, his wife initially said an intruder in black clothing hit her and shot her husband.

During the interview with police, Michele Williams mentioned others that police might want to talk to, including Gregg Williams' ex-wife Kathy Williams, but didn't tell them that her sons disliked her husband.

"I hated him. I don't throw that word around lightly, but I literally hated him," Lee O'Brien said.

"Even though he's not here, I never liked him as a person. He was a horrible human being," Andrew O'Brien said.

After arriving at the crime scene at the Williams' home, police became suspicious of Michele Williams' intruder story. Confronted by police during her interview, Michele Williams changed her story and said her husband committed suicide.

She told police she covered up his suicide to protect the couple's daughter. After five hours in the interrogation room, police released Michele Williams.

When they found out Gregg Williams died, the brothers said they went straight to their mother to be there for her. Andrew O'Brien said that after helping Michele Williams clean up the home where Gregg Williams was killed, she took him outside and asked him to do something for her.

"She said, 'I want you to call a friend, and I want you to do this. I want you to have them go and buy an extra-large sweater ... Wear garbage bags so that the DNA doesn't get on it. Go out somewhere and shoot a pistol with that sweater on,'" Andrew O'Brien recalled.

He said his mother then told him to plant the sweater in Kathy Williams' car and make an anonymous tip to police that Kathy Williams killed Gregg Williams, so they would search her car and find the sweater with gun residue.

"I was like, 'Okay, I'll do it,' and I wasn't going to do it but ... to me it was, 'She's having a mental breakdown,'" Andrew O'Brien recalled. "I just need to just say, 'Okay,' and walk away."

Andrew O'Brien said that when his mother started to change her story about what happened, he became suspicious of her.

"She told me that someone broke into the house and killed Gregg, that the cops made her say it was a suicide," Andrew O'Brien recalled. "I don't even know how many weeks or how much time goes by but then another story comes out, and this new story is, 'Okay, Greg did kill himself.' And ... I was like, 'Why are you lying to me?'"

When his mother asked him if his brother Lee O'Brien could have killed Gregg Williams, he said that was the moment he stopped talking to her.

"My brother, he's got military training. He would not break into someone's house and use their gun," Andrew O'Brien said. "Why would you break into someone's house and take their gun, you know?"

Police said both Lee and Andrew O'Brien had strong alibis. The brothers said they began to realize their mother might be a monster.

"She doesn't even know what reality is any more. She's been lying so long," Lee O'Brien said. "She knew nothing else. Lie after lie after lie."

"You know, you got your mother who was a horrible person, but still she gave birth to you and all that. And you just ... don't want to believe that she's capable of murder," said Andrew O'Brien.

Not confident in their case against Michele Williams, prosecutors offered her a plea deal which required her to plead guilty to tampering with evidence and deadly conduct with a recommended prison sentence of 18 years.

"This is a circumstantial evidence case. There's no doubt about it," Prosecutor Jack Strickland told "20/20."

But while awaiting sentencing, Michele Williams told the national crime television show "48 Hours" that she was innocent and that the intruder she originally told police about killed her husband.

A judge then revoked the plea deal and ordered her to stand trial for Gregg Williams' murder.

Andrew O'Brien took the stand for the prosecution and testified in court about how his mother asked him to set up Kathy Williams as the killer.

"It took me a very long time to decide to do it because after I told them, I said, 'I really don't want to go and testify,'" Andrew O'Brien said. "But I had to do the right thing."

After deliberating for seven hours, the jurors found her guilty of murder and one count of tampering with evidence. But the 60-year sentence is little solace for Andrew O'Brien, who considers his mother dead to him.

"Today I can believe it, and it's because I finally let go of trying to believe that I still have a mother," Andrew O'Brien said.

"I'll never speak to her again, and it's because in order to move on with my life, I need to just cut ties completely."

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