After stabbing his grandmother to death with a kitchen knife, Andrew Arjune Tiyler Johnson stole about $2,000 from her, got high and went shopping.

"This was a cold-blooded, premeditated ... thrill kill," said Clackamas County prosecutor Scott Healy.

Johnson, 18, pleaded guilty to murder Tuesday in Clackamas County Circuit Court and was sentenced to life in prison. He will be eligible for parole in 25 years.

Johnson spent most of his life in Hawaii. In 2011, he was sent to live with his maternal grandmother, Donna Tennant.

Donna Tennant

Johnson's family hoped Tennant, a well-respected, longtime Multnomah County court employee, could help his behavioral problems.

Johnson arrived at Tennant's Milwaukie home around 1:30 a.m. on Dec. 8, 2012. He told police that he had gotten into trouble at school and was upset with his grandmother.

There's no question about what happened next, Healy said. Johnson gave detectives "cool, calm, collected" confession just hours after his arrest.

Johnson said he and Tennant spoke calmly for several minutes. Then she went to bed. Her last words to him: "Good night. I love you."

"This thing in my head is just saying 'kill her, kill her, kill her,' " Johnson told detectives.

He went to the kitchen to get a knife, deciding on a small sharp blade would be less painful and more efficient.

He checked the time as he entered her room. It was 2:03 a.m. when he plunged the knife into his grandmother's neck.

Tennant woke up screaming. Although she was stabbed in the head at least 10 times, Tennant appeared to be alive. Johnson said he strangled her for two or three minutes.

Then he washed his hands and went to bed.

Healy said Johnson ransacked the house and found around $2,000. He bought meth, marijuana and cocaine and splurged on gifts for his friends.

Johnson did not address the court and sat quietly during the 75 minute hearing.

Johnson's mother, on the other hand, offered an emotional and theatrical statement, at times barely able to speak.

Aaliyah Mays, who brought an urn holding Donna Tennant's ashes to the sentencing, apologized to Johnson for physical and emotionally abusing him and failing to act.

"My son was screaming for help and I didn't hear him," Mays said. "No one did."

Mays also shouted obscenities at Johnson's biological father, who was in the courtroom.

Johnson's attorneys, Wayne Mackeson and Diane Rader, had planned an insanity defense.

A defense psychologist concluded that Johnson did not suffer from a qualifying mental disorder.

Healy called Johnson a dangerous sociopath who speculated on what it would be like to kill.

Investigators found that Johnson threatened to kill a cousin, claimed to have slit the throats of homeless men in Hawaii and fantasized about mass murder, Healy said. "He wanted to know what it was like to kill someone."

Healy said there is no evidence that Johnson cut anyone's throat.

Johnson said he considered a mass killing at Milwaukie High School, Healy said. Johnson decided against it. "There were too many exits and a gun would be too hard to get," Healy said.

Circuit Judge Douglas Van Dyk told Johnson he did the right thing by taking responsibility for his deadly deed.

"This is an extremely tragic event," Van Dyk said. "There is so much suffering surrounding this that it defies description."

Van Dyk said there was something the teenager could do on behalf of Donna Tennant.

"Your grandmother would have you redeem yourself," said the judge.

-- Steve Mayes