Steven Nurse never wanted to be a father, but when it happened unexpectedly in his mid-30s family members say he embraced parenthood by taking his son on outdoor adventures to camp, fish and hike. The two of them got along like friends, so no one who knew them could explain why Nurse's son wanted to kill him for several years and finally did in July 2017.

Skye McMillon on Monday appeared in a Travis County criminal district court and pleaded guilty to his father's murder under an agreement with prosecutors that could keep the 25-year-old behind bars for 40 years. Sheriff's office investigators said Nurse died from blunt force trauma to his skull and throat, possibly after being struck by a barbell that they said had his blood on it. His remains were discovered several days after his death in his southwestern Travis County home, detectives say.

Nurse's sister, Karen Mothersbaugh, flew in from Pennsylvania to address McMillon from the witness stand and called his recognition of guilt "a step in the right direction."

She said the most cruel thing he did was withhold an apology and an explanation for a 2011 shovel attack that left Nurse with a serious brain injury that took away cognitive and physical functions. Nurse, who worked for Motorola, was reduced to an entry-level job and was being trained by people he had trained as a manager before he was injured. He struggled to string words together and lost his confidence in social gatherings, she said.

"He loved you more than anything in the world," Mothersbaugh told McMillon.

McMillon, an Eagle Scout who at the time was approaching high school graduation at Lake Travis, told detectives he had wanted to kill his father, and then himself, for two weeks. He had complained to his parents earlier in the school year about mental issues but was told by them to brush it off, Mothersbaugh said.

He spent 16 months in jail before he pleaded guilty to aggravated assault with a deadly weapon. He was given probation and sent to the state's Substance Abuse Felony Punishment Facility, records show. Upon his release from the facility, he was instructed to report to Travis County's Mental Health Support Court.

Defense lawyer Keith Lauerman, who represented McMillon in the 2011 and 2017 cases, called the killing "one of the most unexplainable and saddest cases I've had in 25 years."

A tampering with evidence charge was dismissed as part of the plea deal. McMillon must serve half of his sentence before he is eligible for parole.

After the 2011 attack, a judge told McMillon he could no longer live with his father without the court's permission. That restriction was eventually lifted, and he returned to the family's home on Sandy Beach Road when he was released on probation in 2014. McMillon was jailed the year before on a probation violation for not reporting to his supervision officer.

Concerned about her safety, McMillon's mother, Melissa, left the family and moved to the Beaumont area, Mothersbaugh said. That left Nurse alone with his son.

The investigation into the homicide began after Nurse hadn't shown up to work in two weeks. Deputies came to his home and found him dead on the floor, covered with a comforter, a blanket, towels and what appeared to be salt and foot powder.

Detectives traced Nurse's cellphone and found it with his son in his father's green Subaru Outback in a parking garage at the Hill Country Galleria. McMillon told them he had last seen his father two days earlier when he dropped Nurse off at a hospital, but detectives could not find any record of a hospital visit.

About a month after the arrest, Melissa McMillon was killed in a car wreck along with her sister and nephew. She had written to her son in jail, saying she still loved him, Lauerman said.