A man has been sentenced to six years in jail after he stabbed his violent, socially isolated son to death with a kitchen knife.
Hideaki Kumazawa, 76, called police after stabbing his son Eiichiro, 44, in the neck and chest more than 30 times at his home in Tokyo in June.
He pleaded guilty during a trial at Tokyo District Court.
Eiichiro, who had a developmental disorder and was routinely violent toward his mother, died from massive blood loss. He had been removed from his parents and was living alone in an apartment.
A week before his death, however, he returned home, resumed his violence against his mother and threatened to kill his father, the court said in a ruling.
Defence lawyers said Kumazawa, a former deputy minister of agriculture, forestry and fisheries, feared his son would harm others.
Days earlier, another social recluse, known in Japan as hikikomori, stabbed a number of schoolchildren at a bus stop outside Tokyo, killing two people and wounding 17 others, most of them schoolgirls, before killing himself.
Hikikomori are described as people who have been isolated at home for at least six consecutive months without going to work.
In seeking a suspended term, lawyers said Kumazawa had previously supported his son, despite the violence towards his mother, and killed him in self-defence.
But Judge Tomoyuki Nakayama said the number of stab wounds, and the fact that some were very deep, indicated that it had not been purely an act of self-defence.
Kumazawa said it was his duty to pay for his crime and to pray for his son in the afterlife.
He was sentenced to six years in jail.
According to a government survey in March, there are an estimated 610,000 hikikomori in Japan.
Mostly men, and aged between 40 and 64, many are still cared for by their elderly parents.