Leandro Acosta, 25, killed mum Miryam Kowalczuk, 50, and stepfather Ricardo Klein, but also had sex with their bodies and then ate some of their remains.
He then cut up the rest and burnt it before feeding it to the family's six dogs.
According to Acosta’s lawyer, he killed them because Klein had abused him and his 11-year-old siblings.
Meanwhile police believe he was helped by his stepsister Karen Klein, 22, who had become his lover.
They justified their actions after saying that although the abuse they both experienced was in the past, they discovered to their horror that the dead couple had also started abusing the two children they had together, 11-year-old twins.
They said they had reported the matter to police, and when nothing happened, they had decided to take the law into their own hands.
So they waited until the twins were at school and then carried out the killings at the family home in the city of Pilar, in the east Argentine province of Buenos Aires.
The victim's brother, Roberto Klein, admitted:
"My brother was a difficult man, with little time for his children, there has always been resentment between him and his stepson and I believe this was a long time coming."Acosta’s defence lawyer, Monica Chirivin, said:
"He does not feel remorse and in fact describes a state of relief for a burden he has felt since childhood.He told me that he ate a bit of the corpse.For him it was almost a pleasure."The crime was discovered when a relative of the victim notified the police after he grew suspicious when he was not allowed entry into the family home.
The police found pieces of a female pelvis and vertebra column in an old paint bucket on the terrace.
Several rubbish bags which the couple had taken to a nearby waste ground are also being analysed by the crime squad.