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