McKinney's childhood, too, was less than picture-perfect. His father, a long-haul trucker, was rarely home and eventually divorced McKinney's mother, a nurse who later died as a result of a botched surgery. McKinney received a malpractice settlement of nearly $100,000 after his mother's death. He says he spent most of that money on things like cars and drugs.
McKinney admits to Vargas that by the time he was 18 he had a serious methamphetamine habit.
Despite his strong family life, Shepard had troubles of his own. His mother, Judy Shepard, says her son's problems had started three years earlier during a high school trip to Morocco, where he was beaten and raped.
"It made him pull within himself. He became withdrawn, depression, panic attacks," she said.
Some of Shepard's friends say he was still a troubled young man when he enrolled at the University of Wyoming in the fall of 1998.
Tom O'Connor, known as "Doc," who ran a limousine service and sometimes drove Shepard, said just days before Shepard's death, Matt told him he was HIV-positive and was considering suicide.
One of Shepard's college friends, Tina LaBrie, was concerned that Shepard's depression might be somehow connected to involvement with drugs. "He said 'Everywhere I move, it seems like I get sucked into the drug scene,'" LaBrie told Vargas.
As a heavy user and a dealer, McKinney was well-known with the methamphetamine crowd, according to Ryan Bopp, who was one of McKinney's friends and drug associates at the time. By the fall of 1998, McKinney had blown through his inheritance and was now the parent of a new baby with his girlfriend, Kristen Price.
"I think he was really torn because it is the desperation of getting your fix or taking care of your family," Price said. In the days leading up to the attack on Shepard, she said, McKinney was using methamphetamine every day.
Bopp, who says he left Laramie and the drug world behind six years ago, told "20/20" that he and McKinney had been on a drug binge in the week leading up to the attack on Shepard.
"Aaron and I had been awake for about a week or so prior to this whole thing happening ," Bopp said. "We were on a hard-core bender that week."
Bopp also admits that a week before the murder he was so desperate for methamphetamine, that he traded McKinney a .357-Magnum pistol in exchange for one gram of methamphetamine. McKinney would later use that weapon to beat Shepard.
McKinney told Vargas he set out the night of Oct. 6, 1998, to rob a drug dealer of $10,000 worth of methamphetamine. But after several attempts, McKinney was not able to carry out his plan.
Henderson said he thought if he could keep McKinney drinking, he'd forget the robbery plan.
But according to McKinney, when he encountered Shepard at the Fireside Lounge, he saw an easy mark.
McKinney told "20/20" Shepard was well-dressed and assumed he had a lot of cash.
Shepard was sitting at the bar, McKinney recalls. "He said he was too drunk to go home. And then he asked me if I'd give him a ride. So I thought, yeah, sure, what the hell," according to McKinney.
All three got in the front seat of McKinney's pickup, and Henderson took the wheel. McKinney told police that at some point Shepard reached over and grabbed his leg. In response, McKinney said, he hit him with his pistol. "I was getting ready to pull it on him anyway," he said.