A court has heard a Queensland teenager who brutally murdered her grandfather was inspired by the violent television series American Horror Story .

Then 19-year-old Brittney Dwyer travelled to Adelaide with an accomplice in August 2016 and planned to rob her grandfather Robert Whitwell, 81, of his life savings.

The elderly man had invited his granddaughter into his Craigmore home, in Adelaide's north, and the pair had looked at family photos before she stabbed him in the throat and chest.

Dwyer has pleaded guilty to murder and is awaiting sentencing.

In a pre-sentencing hearing on Wednesday, Dwyer's lawyer Craig Caldicott asked the judge to show leniency because of his client's age.

He said there was little he could say by way of explaining Dwyer's crime, except that she had a borderline personality disorder, she was motivated by money and the American television mini-series American Horror Story.

Brittney Dwyer murdered her grandfather, Robert Whitwell. (9NEWS)

Justice Kevin Nicholson described the murder as pre-meditated, callous inexplicable.

"I'd have to consider that your client is a dangerous person. At age 18, to go from zero to pre-meditated murder suggests a very worrying person," he said.

"She has an extreme lack of empathy... it's very hard to measure genuine contrition. It may be she's simply not capable of it. It's a very worrying factor."

Accomplice pleads for mercy

Dwyer carried out the killing while her accomplice Bernadette Burns, 22, sat in the car and, through text messages, encouraged her to do "whatever it takes" to steal Mr Whitwell's life savings.

She's pleaded guilty to a charge of "statutory murder" which means she accepts an intentional act of violence could have been carried out when committing the robbery, but that she did not hold a murderous intent.

Burns' lawyer Anthony Allen said his client was not as culpable as her co-offender.

He said she should receive a non-parole period significantly lower than the mandatory minimum of 20 years for murder, because she was not the one who held the murder weapon.

"She did not have the intention to kill or do grievous bodily harm," he said.

He said she was deeply remorseful and regretted her actions.

"She feels great shame and sorrow for what she did," Mr Allen said.

"She knows what she's done is unforgivable but through me she says she is sorry."

Mr Whitwell's brother Geoffrey said the killing was tragic and senseless. (9NEWS)

Family members tell of their heartache

In victim impact statements read to the court, family members of Mr Whittwell and Dwyer told how their lives have been forever changed by the crime.

Ryan Whitwell-Dwyer asked the judge to impose the maximum penalty on his sister.

"My grandfather Robert Whitwell was a proud and giving family man, who always went above and beyond for anyone in need," he said.

"Brittney, I was always your number one supporter, I was always there for you, you've thrown this in my face," he told his sister.

"There were countless moments you could have come clean but instead you chose to betray me, your own brother."

Dwyer's mother Tonya Dwyer said she lives with the crime every day.

"Images of what my father went through in the last minutes of his life consume me with sadness," she said.

The victim's brother Geoffrey Whitwell said he has lost his best friend.

"I sit at home at night and know that I will never hear the phone ring and hear my brother on the other end. You have completely devastated me," he told Dwyer.

He said he had comforted Dwyer at his brother's funeral - before she'd been identified as his killer.

"You are a cruel, evil, dangerous person with no regard for human life and you deserve the longest non-parole period the law permits."

Another brother, Peter Whitwell said: "I'm haunted by nightmares and have sleepless nights thinking about Robert's last moments."

Burns and Dwyer will be sentenced next month.