What provoked a man who was a "good student" at junior school, and bright enough to get into one of the most competitive high schools in his area, to commit an apparently random act of mass murder?
Did Japanese society play a part in Kato's alleged actions?
Tomohiro Kato, arrested after his Sunday lunchtime stabbing spree that left seven dead and 10 injured, has been handed over to prosecutors and is likely to face the death penalty.
Perhaps that is what he wanted.
The police say that he went to Tokyo's Akihabara shopping district to kill people, saying he was tired of life.
In the Japanese newspapers the pundits all have their theories as to what prompted the attack.
He is "a sociopath who blames society for his unstable life as a temporary worker", according to a criminologist quoted in one article, who feels that "many young people are selfish and immature and such violence is a manifestation of this".
This kind of response could be characterised as "it was a problem with him, not something we did wrong". But is that really good enough as an explanation, or was he failed in some way by Japanese society?
'Falling through the cracks'
Professor Jeff Kingston, a Japan watcher from Temple University in Tokyo argues that recent cases of deranged young men committing random acts of violence here suggest that the public health system in Japan does not provide adequate support for people with mental illness.
"There is a social stigma attached to mental illness," he says, "and in addition to that doctors are reluctant to refer patients for psychiatric counselling, so this is probably a far too common case of an individual falling through the cracks in the system."
Kato was on a temporary contract. Police say they now believe he was unhappy at work and that provoked his murderous assault.
He threw a tantrum in front of colleagues at the factory where he was working on a temporary contract a few days before the attack because his work clothes went missing.
He thought this meant he was going to be laid off.
We know that Kato gave numerous indications that he was troubled, posting messages online
But those who worked with him report that in general there was nothing extraordinary about him.
As a "freeter" - as temporary workers are known here - he would not have had access to the kind of counselling or support services that might have been available to a full-time employee.
He probably would not even have worked with the same people on a regular basis, so it was less likely they would have noticed there was anything wrong.
We know that Kato gave numerous indications that he was troubled, posting messages online on bulletin boards expressing rancour, rage and alienation. But these were anonymous posts.
Professor Hirokazu Hasegawa, a clinical psychologist from Tokai Gakuin University says this decision to express his pent-up frustration on the internet suggests that perhaps he had trouble talking to people and communicating what he truly felt.
People pay their respects to victims of the attack at a makeshift altar
He agrees that the system here is failing those who need help for mental illness.
"There are not enough qualified specialists available," he says.
"The chance of somebody with a serious problem actually getting treated by a qualified and perhaps more importantly experienced specialist is very small here. It's hard to get real help."
Professor Hasegawa also criticises what he calls the "pent-up suffering of Japan".
He argues that Japanese parents treat their children differently from parents in countries like the United States or the United Kingdom.
In Japan parents tend to regard their children as possessions, as part of them, and as a result often they don't see them as individuals or respect their rights as individuals.
Children are expected to follow closely what their parents think and say and to try to behave or to conform to what their parents see as appropriate behaviour.
The attack has stirred intense public speculation
Negative feelings such as anger or frustration build up because it is much harder to find a way to express them.
Inevitably the politicians are now being pressed for their views on what if anything can be done to prevent such crimes.
Japan's Education Minister, Kisaburo Tokai, says he is considering holding talks with "experts in brain science about the state of children in Japan."
At the same time, inevitably perhaps, a debate is under way about whether Japan is becoming a more violent society.
The Asahi newspaper reported that there have been 67 random killings in Japan in the last 10 years, on average between three and 10 each year. Last year there were eight. The year before there were four.
So far this year, according to the newspaper's definition there have been five attacks - three major killing sprees and two where an individual was targeted, apparently at random.
So this year is on target to be at the higher end of the scale, but not unusually so.
The newspaper claims that in the past drugs were often blamed for prompting random attacks.
These days, it says the motive is more likely to be a grudge against society. It is hard to be certain whether this is the case or not.
However, as Professor Kingston points out, Japan should be grateful for its strict gun control laws.
"If Kato had had an automatic assault rifle the mayhem in Akihabara would have been far more devastating."
And the fact remains that by most measures, Japan is still much safer than many other countries in the world.
"I would rather take my chances on the streets of Tokyo than London, New York or Rio de Janeiro anyway," the professor says.
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