He is accused of murdering Chloe over money, the BBC reports, having argued with her over an overdue payment of £150 ($347) - a tenth of the £1500 ($3473) she had owed him.
He was more than £8000 ($18,500) in debt himself - generated predominantly from his gambling habit.
The Times reports Martin told police he could not explain why he had allegedly attacked his stepdaughter.
“I don’t know, I’d just had enough, it’s just constant, money, money, money, go out, go out, go out...”
Prosecutor Julian Evans described how Martin attacked Chloe with a hammer before stabbing her eight times.
While he was no longer married to her mother, Elaine Sturges, the 30-year-old reportedly still saw him as a father figure, and her four children considered him a grandfather.
He went on to murder Chloe’s husband when he returned home, stabbing him four times before strangling him to death.
Martin then moved the bodies into two rooms of the house, removing the handles from each door.
After picking the murdered couple’s children up from school, according to People, Martin texted Sturges, then turned himself in for the crime.
“Elaine I’m so sorry, I can’t believe what I’ve done, I know everyone hates me anyway especially the boys, I hate myself anyway and please, please look after the children really well,” he wrote.
“I’m just about to walk into the police station then that’s my days over and good job too, I know it’s going to mean nothing but I’m so sorry, don’t take the children home x.”
Martin has a history of mental health issues and has reportedly attempted suicide several times, Metro reports.
He was not taking his medication at the time of the murders and had stopped sleeping more than one or two hours a night.
His state of mind is the key question at issue, with his psychiatrist, Dr Seena Fazel, telling the court the killings had not been intentional, according to People.
The trial is set to continue for another two weeks.
