Brittney Dwyer was 19 years old when she stabbed 81-year-old Robert Whitwell in his Craigmore home in Adelaide in August last year.
The South Australian Supreme Court heard Dwyer, now 20, planned to steal her grandpa's $100,000 life savings, which he kept in his locked shed.
After stabbing him four times, Dwyer then inexplicably did the washing up as her grandpa bled to death. After failing to get the shed open, she left with just $1000 cash.
She has now been sentenced to a minimum 21 years in jail, with the judge describing the crime as callous and cold-blooded.
Dwyer drove to Adelaide with her friend Bernadette Burns, now aged 22, who encouraged her to commit the murder through text messages, saying she should 'harden up and just do it.
Burns was also sentenced to life imprisonment for a statutory murder charge, with a non-parole period of 13-and-a-half years.
Sentencing the pair, Justice Kevin Nicholson said Dwyer was motivated by greed and an unnatural interest in seeing people die.
'This murder was brutal, callous, cold-blooded and dispassionately planned,' he said.
Tonya Dwyer, hasn't spoken to her daughter in more than a year, said she still struggles to comprehend how her daughter could kill her father.
'I have lost two people, I have lost my dad and I have lost my daughter, and I don’t know if I will ever understand the whole story,' she said in an exclusive Sunday Night interview.
'What are the signs I missed? What did I miss? What didn't I see? What didn't I pick up on?' It was a person that I trusted, that I brought into the world, that I raised to be a good person.'
She continued, 'It’s like a nightmare that I never wake up from. Every day I think, ‘when is this nightmare going to finish?’ And it doesn’t. It’s my life.'