Henri van Breda, 23, has denied murdering his 21-year-old brother Rudi and parents Martin, 54, and Teresa, 55, and leaving his sister Marli struggling with near-fatal injuries to her head, neck and throat.
During the trial, van Breda told the court that an intruder had entered the family's house late at night on an exclusive estate in Stellenbosch, outside Cape Town.
But in his summary, Western Cape High Court judge Siraj Desai raised multiple questions about the defence case.
"The court is in agreement that it is highly unlikely that the security was breached by an intruder from outside the estate," Desai said.
"The accused could not say how the security was breached (and) confirmed that he never felt insecure, unsafe or threatened while living on the estate."
During his evidence, van Breda had claimed that he was in the toilet when he saw the masked intruder attacking his family.
He said he had survived the attack with superficial stab wounds inflicted after he wrestled with the attacker who was trying to slit his throat. He said he then fell unconscious.
The state insisted the wounds were self-inflicted.
'No grudges against family'
"The accused testified that he was unaware of anyone with grudges against the family," Desai said.
"The accused testified that he could not think of a reason... why the intruder did not simply remove valuables from the house.
"The accused denied having any serious arguments with his family and stated that there was nothing out of the ordinary the night before the attack."
Desai said the axe used in the attack was kept in the house.
He also questioned why van Breda did not immediately call security or emergency services after the attack in January 2015.
"If the accused's intention was to be helped as soon as possible, it does not explain why he first called his girlfriend, a minor schoolgirl residing in a hostel, several times from his mobile phone," Desai said.
Van Breda's family had relocated to Australia many years ago before returning home to South Africa in 2014.
Believing he was about to be arrested, van Breda handed himself in to a local police station 18 months after the killings.
The trial generated fevered interest over a privileged son allegedly unleashing a savage attack on his family, whose fortune, derived from property, has been estimated at $16 million (13.61 million euros).
Van Breda's claim of an intruder had echoes of Oscar Pistorius's defence that he thought a burglar was hiding in the toilet when he fired four times through the door killing his girlfriend.
In a tape of van Breda's call to emergency services, he said his family members were "bleeding from the head" before he appeared to giggle briefly.
The judge will continue his summary later Monday.
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