Sheffield Crown Court heard of the "toxic home life" of the boy and his brother, who were just ten and 11 when they subjected two others, aged nine and 11, to a violent 90-minute ordeal in Edlington, South Yorkshire, last April.
A barrister outlined how one of the boys watched the gruesome Saw movies when he was ten, and had access to pornographic DVDs. He said he also smoked cannabis from the age of nine and drank cider.
The court has already heard that the victims were strangled, smashed with bricks, forced to eat nettles, stripped and forced to perform sex acts on each other.
Both boys told police their arms had been aching following the attack, and claimed they would have continued if it had not been for this.
The brothers have admitted causing their victims grievous bodily harm, a sex charge and robbery. They are going through a sentencing hearing, which is expected to conclude today.
Yesterday, the judge heard the elder brother said he attacked the boys because he was bored.
When asked by police why he had attacked and injured the boys,
he had replied: "Don't know. 'Cos there were nowt to do."
Peter Kelson, QC, representing the elder brother,
described how the boys' mother had told psychiatrists about years of domestic violence perpetrated by the boys' father on her and her five sons.
The lawyer described one incident, in which her partner said he would "take a knife to her and slice her face to bits".
He added: "These threats were made in front of the children."
He said the older brother would drink cider, had ten cigarettes a day and from the age of nine had smoked cannabis grown on his father's allotment.
The case continues.