"I never want to be let out. I will kill. I know I will," Gardner, 31, tells 48 Hours Mystery for Saturday's episode. "I am the type that needs to be locked up forever. I am an animal."
In a plea deal sparing him the death penalty, Gardner admitted he strangled Chelsea King, 17, whose body was found in a San Diego-area park, and fatally stabbed Amber Dubois, 14, in a remote location near Pala, Calif.
He also admitted attacking a female jogger who escaped only days before he jumped King as she jogged through the same park.
Although King's body was found within a few days, Dubois's disappearance on her way to school remained a mystery for 13 months as her family prayed for the best, feared the worst, and wondered whether Amber's own stepfather was responsible.
Gardner recalls for 48 Hours Mystery the Dubois abduction in detail and says he hopes he's killed in prison.
"Eventually someone will come and get to me," Gardner says. "That will be a release for me. … It's either going to be that or I end up killing myself, one of the two. I can't stand to be confined. I'm like an animal. Just like they said. I'm an animal."