An obsession with graveyards and "dark things" set a teenager on a macabre path which led her to murder her grandfather, an elderly pensioner who had treasured her as a child, her mother says.

Brittney Dwyer calmly washed dishes only metres away from where her grandfather, 81-year-old Robert Whitwell, slowly bled to death at his Adelaide home in early August last year.

Outside in a car, its engine still warm from the long drive from Brisbane, Dwyer's accomplice Bernadette Burns sat, applying makeup and sending text messages prompting Dwyer to "harden up" and commit murder, news.com.au reports.

Burns faces deportation to New Zealand once released from jail as she was born here and is not an Australian citizen.

Dwyer's mother and daughter of Whitwell, Tonya Dwyer, told Channel 7's Sunday Night that she had watched as her daughter grew from a bubbly child to a sullen teenager who self-harmed and was obsessed with violence and death.

Bernadette Burns drove Brittney Dwyer to Adelaide where Dwyer killed her grandfather. Photo / Facebook

She and her best friend Shelby Lee Holmes had a fascination with knives and violence which culminated in Dwyer stabbing and killing Whitwell.

"I think they are a little bit dark, as in they would do things in cemeteries and go to cemetery tours and dark things," Tonya Dwyer said.

"There were some things with blood and knives. I think they had a bit of an obsession with some dark stuff."

Holmes, also speaking to Sunday Night, denied the interest was any more than a typical teenage behaviour.

"It was just what any other normal teenager would be interested in, like horror movies and things like that," Holmes said. "Nothing alarming."

Tonya Dwyer says she watched her daughter grow from a bubbly child into a sullen teenager. Photo / Channel 7

Tonya Dwyer said she was still "furious" with her daughter for making accusations Whitwell had sexually abused her as a child as a cover for a failed plot to steal his money which ended in murder.

"I was furious and that's the reason why I would still slap her in the face," she said. "All that anger that is inside."

Holmes had accompanied Dwyer on the long drive from Brisbane to Adelaide in April 2016 in an attempt to steal more than A$100,000 Whitwell had hidden in his garden shed.

The attempt failed with the pair escaping empty-handed but not before Dwyer warned that her grandfather "might have to die" if they were to get the money.

Four months later Dwyer was back on the road to Adelaide, this time with 22-year-old Burns who was in a self-destructive cycle after losing custody of her young daughter.

Wearing gloves and concealing a knife Dwyer knocked on Whitwell's door.

Robert Whitwell bled to death at his Adelaide home.

They spent some time together before Dwyer stabbed him several times to the neck as they were heading to the front door.

He bled to death, despite a token plaster Dwyer applied before going to the sink to do the dishes.

Dwyer and Burns left Adelaide with a little over A$1000 in cash after having once again failed to find the life savings.

Dwyer drove straight from Adelaide to her mother's house in Brisbane where she had only just learnt of the fate of her father.

"Little did I know she was still driving back from Adelaide," Tonya Dwyer said.

"I told her the police have just rung and Poppa has just passed away. She just burst into tears, hysterically burst into tears.

"I said I need to you come with me and fly down to Adelaide."

Bernadette Burns faces deportation to New Zealand once released from jail as she was born here.

In the days that followed Dwyer grieved alongside the family of the man she had murdered with her mother only thinking that her chain-smoking during that time was out of character.

An anonymous tip-off would lead police to arrest Dwyer and Burns and it was not long before both pleaded guilty to the crime.

"I've lost two people, I've lost my dad and I've lost my daughter," Tonya Dwyer said.

"I am not sure I will ever understand what happened."

Dwyer and Burns are set to be sentenced tomorrow.