He cried at his murdered uncle's funeral in 1997, writing years later that he didn't understand why it bothered him so much. He described a mass murder in Fresno, Calif., in 2004 as "sick, sad and tragic."
Now, the Internet-obsessed loner is accused in one of the most grisly, tragic and bizarre murder cases in Oklahoma history. The FBI and police disclose in court affidavits that he confessed.
Underwood, 26, is accused of beating and smothering his 10-year-old neighbor, Jamie Rose Bolin. Police and a prosecutor allege the grocery store stocker killed for his own sexual gratification and to fulfill a fantasy of becoming a cannibal.
"My fantasies are just getting weirder and weirder," he wrote in an Internet diary in September 2004. "Dangerously weird. If people knew the kinds of things I think about anymore, I'd probably be locked away. No probably about it, I know I would be."
On that Wednesday, April 12, "whoever walked by his apartment was going to be a victim," Police Chief David Tompkins said. "It didn't matter."
The prosecutor, District Attorney Tim Kuykendall, said, "We believe he purchased items months in advance in preparation of kidnapping and killing someone. ... Other people had been targeted and considered ... children, 5-year-old boys, adult females."
Jamie, a likable, cute, red-haired, freckled girl, was unlucky. Passing by the apartment with her purple bike, she was lured in, police say.
She knew better, officials say. Her father, auto mechanic Curtis Bolin, had told her not to go in a stranger's apartment.
She had even told Underwood the night before her death she would get in trouble with her father if she came into his apartment, the prosecutor said. Underwood offered her that night the use of his phone. She was downstairs then using a pay phone to order pizza.
Still, the next day, she went into Underwood's apartment of her own free will, authorities said. He had befriended her before, putting his pet white rat, Freyja, on his shoulder and letting her touch it.
She was watching a cartoon when he began hitting her in the head with a wooden cutting board, the prosecutor alleges.
"We believe that she screamed numerous times. She was crying," Kuykendall told CNN.
Unlike most murder cases, police and prosecutors revealed many details to the media, saying the truth was better than rumors.
They disclosed she said she was sorry as Underwood began hitting her. She pleaded to be let go, promising not to tell on him. They said he smothered her with his hand and duct tape.
After her death, officials allege, Underwood tried to cut off her head with a decorative dagger, leaving deep saw marks in the neck. The body otherwise was still intact.
Officials also allege he may have sexually molested her corpse.
Jamie's body was found in a large, blue taped-shut plastic tub in the bottom of Underwood's bedroom closet. A white, striped towel was in the container to soak up the blood. Her clothing had been removed.
FBI agents went with Underwood to his apartment April 14 to see if Jamie was there. They questioned him after he was stopped at a checkpoint. The apartment manager, Tim Bayer, told the FBI he'd heard Underwood was the last person known to have talked to Jamie.
FBI Agent Craig Overby disclosed in a court affidavit that Underwood at first claimed comic books were in the container.
Overby began looking inside and saw a sleeveless blue top -- the same kind Underwood said Jamie was wearing when he last saw her. The agent wrote that Underwood then said, "Go ahead and arrest me.