"I like to watch the eyes fade, the pupil fade. It's just like setting their soul free," Sells said at the time, without showing emotion. "I don't have an on-and-off switch. I'm just after that drug. I'm after that feeling."
One of the more horrific crimes Sells has confessed to committing was the slaying of 30-year-old Eileen Dardeen and her family.
Dardeen was seven-and-a-half months pregnant when Sells allegedly beat her to death. During the attack, she went into premature labor and gave birth to a baby girl.
Investigators determined the newborn was alive when she was born, but was later found bludgeoned to death, as was Dardeen's 3-year-old son.
Her husband's body turned up a few days later. He had been shot three times in the head.
Investigators involved with the Dardeen case said if Sells were not already sitting on death row in Texas, they would have arrested him for this crime.
Sells blamed his murderous rage on having an abusive childhood.
"I didn't want them to live through the pain I lived through," Sells told ABC News, explaining why some of his victims had been children.
Sells has left only one known survivor during his terrifying rampage.
Krystal Surles was just 10 years old when Sells tried to murder her while she was sleeping over at her friend Katy Harris' house in Del Rio, Texas.
Asleep on the top bunk, Surles awoke in the middle of the night to Sells killing her friend.
Though it happened years ago, Surles told ABC News in an interview that it was a night that will remain burned in her memory.
"He had a hand on her mouth and the knife on her neck, and she's looking at me, at the bunk bed," Surles told ABC News at the time. "He just cut her throat, and she fell to the ground."
Frozen with fear, Surles said Sells then walked towards the door, about to shut off the light, when he looked around the room one last time.
"That's the first time he noticed me," she said. "He ... didn't hesitate at all. I mean, just shut the door, came right back towards me with the knife."
"The only thing that he said is, 'Move your hands' ... and he reached over the top bunk and ... cut my neck."
Sells severed Surles' windpipe and grazed her ceratoid artery. She played dead on the floor in a pool of blood until she thought Sells had gone, then she ran for help at a neighbor's house.
Surles later identified Sells as her attacker and as the man who had brutally killed her friend, which finally put an end to his killing spree. He was later found guilty.
Sells told ABC News in 2010 that he still remembered the one that got away.
"There's not a day goes by that I don't think of her," he said at the time.
The Associated Press contributed to this report.