There had been no adults in the house for a while. Tiffany's father had moved away and let her stay there alone to finish her senior year of high school. Police believe Tiffany's boyfriend, Precella, was selling party drugs like ecstasy and marijuana to Clear Lake students.
Now Christine told police that she and Snider went to the house on the day of the murder because Snider wanted to score drugs. But that didn't explain everything, for the detectives.
"Ultimately, it never made sense," said Harris. "I mean, you look at the brutality of the crime. There was a lot of rage and anger behind these killings."
In her police interrogation, a distraught, chain-smoking Christine Paolilla said that when she and Snider parked near the house, it became clear Snider was up to no good.
"I was right behind Chris," she told an interrogator. "I stayed behind him, like, the whole time because I felt so bad, I was just so scared."
The detective asked what she felt.
"He was gonna, you know, shoot me," she replied. Once the two got inside, she said, Snider became aggressive.
"Chris started arguing with Marcus and it was getting loud," she said. "And that's when I heard the first gunshot. I wanted to run but I couldn't, I felt so scared and I felt so sick. ... And then it felt like I kept hearing like the bubble-wrap noises, like pop, pop, pop, pop, pop, pop."
Christine said Snider had forced her to carry a gun. Then, she said, he made her fire it.
"So the gun was in your hand ..." an interrogator asked.
"He was holding on to it, too," said Christine. "And I was scared, and I was crying, and like, uh, I had made the gun go off, not purposely ... a million times."
Harris summed up her defense.
"In essence, she claims Chris Snider killed all the people and she just held the gun," he said.
Christine said Snider threatened to kill her and her family if she talked, so she composed herself and reported for a work shift at Walgreens.
"I was so scared and ... at the time I felt that if I didn't do what [Snider] was going to say, he was going to shoot me," she said.
"There's been times where he'd get almost satanic talking about people, saying, 'I wonder what I'd be like to kill someone.'"
Christine Paolilla began her trial on murder charges with the defense that her boyfriend at the time, now dead, was responsible. A Texas jury would decide her fate.
For the story of Christine Paolilla's trial on murder charges, visit the "20/20" Web site Thursday. For the full story, watch "20/20" Friday at 10 p.m. ET