Bombshell Witness Takes Stand at Woman's Murder Trial

Part 3: Andrew O'Brien testifies for the prosecution at his mother's trial for her husband's murder.
7:14 | 10/18/14

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Transcript for Bombshell Witness Takes Stand at Woman's Murder Trial
Reporter: At the tarrant county courthouse in ft. Worth, Texas, it's finally time for truth and consequences. Michele Williams is on trial for the murder of her husband Gregg. This story continues to evolve. Reporter: The media following every twist and turn in the murder case. The motive was money. Tried and true, age old, money. Reporter: A fraud examiner says the Williams finances were mismanaged. Lots of problems here, right? They tended to be living beyond their means. They both had their respective jobs in this case. And his job was to make money, and her job was to spend money. And they were both very good at their jobs. Reporter: And prosecutors lay out their theory. Gregg and Michele fought over money that night. Gregg hit Michele or threw something, causing that goose egg on her cheek. It's our suspicion that this is an injury that was caused in a fracas with her husband, earlier in the evening, that led to, led to his demise. Reporter: Provoked, Michele enacted a plan that would get her revenge, and riches. Prosecutors say after Gregg fell asleep, Michele shot him dead with his own gun, then arranging the evidence and cleaning up the crime scene to look like the killer was an intruder. Michele brought to tears as gruesome photos of her dead husband are shown. Prosecutors are so successful in painting Michele as a pants-on-fire caliber liar. She lied about it, she lied about it, and when she finished lying about it she kept on lying about it. Reporter: Her defense attorneys are forced to make a startling admission in open court. Probably, at this point, you've made up your mind and rightly so that Michele, she lies. Reporter: But the most devastating witness to take the stand. It's her own son. Her own adult son. It's Andrew o'brien. Reporter: He testifies his mother had concocted yet another whopper to save herself. She asked him to set up Gregg's ex-wife Kathy Williams as the killer. Michele asked me to call some friends and have them frame Kathy. She said, "I need you to call a friend, tell 'em, go to Walmart. Buy an extra large sweater. Take that sweater out the woods somewhere. Fire a pistol so that the gunpowder gets on it.'" Reporter: And to plant it in Kathy's car where police would find it. You were brought up to the witness stand to testify about your mother, how did you feel? God, that was probably the worst feeling I've ever had, when they asked me to point to Michele Williams. I could not make eye contact. Reporter: As the defense presents its case, Michele Williams' court appointed attorneys say Gregg was actually worth more to her alive. Gregg was Michele's golden goose. He brought in over $500,000 in 2010 and over $800,000 in 2011. So what was her motive? Reporter: So how did he die? They go with Michelle's story number two, remember, that was the suicide theory? Gregg took his own life. That's what the physical evidence supports. What do we know about Greg Williams? We know that he, he was competent in handling a firearm. We know that he was using steroids. Reporter: Michele's attorneys argue Gregg was driven to suicide by a plague of problems. Money trouble. Issues with his teenage daughter from a previous marriage and the recent suicide of a close friend. Nonsense, according to Gregg's mother. She says he was loving life. He was very happy and bubbly and telling me all about the plans he had for the new house they were buying. Reporter: Prosecutors question whether suicide was even physically possible. Crucially, a gun expert testifies Gregg was not, as you might expect in a suicide, shot at close range. The muzzle was further than 6 inches and closer than 24. This is 24 inches. Reporter: Prosecutors demonstrate the acrobatics required to have pulled the trigger from that distance. Does it make any sense to you at all, that somebody would lay back in their bed and hold this gun 11 inches away from their own head and fire into their head? How do you get your hand to do that? How do you cock your wrist to do this? Reporter: The prosecution seemed very convincing. People who commit suicide don't do it that way. He was a bodybuilder. I'm sure this was a pretty common stance for him. So, it wouldn't be -- it's not farfetched that he would've just done this. Reporter: Another problem with the suicide theory, pointed out to us at the crime scene. Although Gregg's left hand is in view, his right hand, his shooting hand, was tucked under the covers. So he would have had to shot himself and then put his hand under the comforter? Correct. Reporter: In their final defense of the suicide theory, Michele's attorneys try to explain why she concocted the intruder story and staged the crime scene. She's not a career criminal, ladies and gentlemen. This is not something she had time to think about. She panicked. She was working, like I said, to protect her baby girl Mikayla who was in the other room. She has tried to pull the wool over the eyes of family and friends and police and prosecutors and courts, the media, the community and ultimately, you. Don't let her get away with it. Reporter: After all Michele's pinocchio moments, the intruder in black. The suicide theory. The imaginary twins. The jury comes back, and hits Michele right between the lies. Mrs. Williams, if you would stand please. The verdict says we the jury find the defendant Michele Marie Williams guilty of the offense of murder as charged in the indictment. Reporter: Michele Williams who had once been offered a plea deal of 18 years, is now sentenced to 60. Do you think she could've gotten away with it? I think she could've gotten away with it. If she had come up with the, with the suicide story, and stuck with the suicide story, she could've gotten away with it. Reporter: What do you think was the one thing that kept her away from freedom? The fact that she couldn't keep her mouth shut. Reporter: Instead, Michele is serving what amounts to a life sentence. It's little solace for Andrew who already considers his mother dead to him. Would you say you have a mother? No. I don't like to even consider her my mother. I had someone who gave birth to me and that's it. Reporter: But he acknowledges his twisted relationship with his mendacious mother has given him at least some direction. She taught me exactly what not to do in life. And I've learned pretty well from it. Reporter: So you learned from example from her? Right. Reporter: What not to do? Exactly. And that's what I focus on, is my family and my kids. And giving them everything I didn't have. So, what do you think about that mom? Matt is live tweeting with the rest of us tonight. Use #abc2020.

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

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