Here be monsters. The phrase was used by early cartographers, and it still captures the imagination. It was a vivid excuse for ignorance, and also a flamboyant plea for it: "We have no idea what lies beyond our known world, and that frightens us beyond reason. It should frighten you too." Now that the world, we like to think, is entirely within our comprehension, the notion seems quaint. Yet when a story comes along to test our understanding of what it is to be human, the old stop-sign is brought out of retirement: "Here be monsters."

For 24 years Elisabeth Fritzl lived beyond the known world, in a fetid dungeon built for the purpose by her father, where he could rape and abuse her whenever he wanted to, impervious to the unspeakable enormity of the torture he was inflicting on her, and on the three of their seven children who subsisted with her underground.

At Josef Fritzl's trial this week his defence lawyer, Rudolf Mayer, argued thinly that this singularly aberrant behaviour did not make Fritzl a "monster". Yet in the event, even Fritzl seemingly came to realise that there was no defence for what he had done. Yesterday he dropped his plea of not guilty to murder (of Michael, a baby twin who had died at three days old) and slavery. He says that until he saw his daughter's video-taped evidence in court, he did not realise how cruel he had been. He says he is sorry.

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It feels comfortable to dismiss Fritzl's regret and his sorrow as a sham, some demented last-ditch effort at obtaining the leniency of the court. It feels less comfortable to contemplate the idea that there is still, in some little-used part of his personality-disordered brain, a capacity, however slight, for critical self-examination, and a measure of comprehension of what he has perpetrated. Human or monster? It is, oddly, much less frightening, much less threatening, to dismiss Fritzl as the latter.

Mayer's contention was that Fritzl had been a victim himself, of his own mother's controlling violence. In abducting and imprisoning Elisabeth when she was 18 years old, Fritzl had really believed that he was keeping her safe from the dangerous temptations of a wicked world, and had really believed, when the babies came, that he was creating a second, superior family. A real monster, Mayer contended, would simply have killed all seven of the children he had fathered with his daughter, because they were of no use to him. If he had done that, he would never have been caught.

Everyone understands that it was a necessary formality for Fritzl to have a "full and fearless" defence. Mayer is well-known for taking the most hopeless and foul of cases. It is acknowledged that somebody has to. But the really disturbing thing about Mayer's defence is that it is not without a smidgeon of verité. It cannot, even now, be entirely dismissed.

Why did Fritzl take his 19-year-old daughter, Kerstin, to hospital when Elisabeth begged him to? Why did he not merely tell her that he had, and kill her? After all, how could this girl whose existence was not known, be traced? And how did Elisabeth persuade Fritzl, after 24 years of incarceration, that he must allow her to respond to televised pleas for the mother of Kerstin to come forward, in order to assist with her medical treatment? Did he have a flicker of sympathy for his tortured daughter, and a twitch of respect for her maternal dedication? If he did, this makes his cruelty even harder to understand. He may really have believed, grotesque as it may be, that he was being cruel to be kind.

And why did Fritzl not just kill the other three babies, those who were released to the surface, brought up by Fritzl and his wife, Rosemarie, under his pretence that they had been dumped on the doorstep by Elisabeth, who had run away with a cult? Perhaps it was all sheer narcissism, an inability to destroy his own flesh and blood, and a belief that he could get anyone to believe his outrageous lies, because they had always been believed before.

Yet if this was a man who baulked at the annihilation of his children, how could he not have baulked at his treatment of his daughter Elisabeth? Her living annihilation was a fate that one can only begin to imagine before deciding that it was worse than death. One can only assume that Fritzl really believed that what he was doing to her was comparatively benign, compared to killing.

Of all the crimes Fritzl is guilty of, the murder of the baby, Michael, carried the gravest mandatory sentence. Yet who does not look at the list of horrors perpetrated by this man, and think that letting a baby die for want of post-natal attention was by no means the greatest of them? This case suggests all too graphically that there are worse things a human can do to another human than murder, things that we prefer not to countenance, that lurk behind a psychic sign that says: "Here be monsters."

Fritzl, except by omission, formally heeded the moral convention that forbids the taking human life. He knew where to stop, or thought he did. Even Fritzl had his standards, standards that are enshrined by Austria's sentencing guidelines, and those of other countries. Criminal justice systems are not designed with men like Fritzl in mind. He is beyond normal comprehension.

Many people, notably Natascha Kampusch, who was abducted at the age of 10 and held for eight years in a dungeon by Wolfgang Priklopil, suggest that Austria's harbouring of not one but two such awful cases is connected somehow to the Second World War. In Fritzl's case, the connection might well be that he considered his dungeon to be something akin to his own one-man totalitarian state, and his incestuous family its citizens.

The totalitarian horror can be understood as what happens when ideologically driven leaders decide that anything is justified because it is all done in pursuit of what they see as a perfect society. When Mayer talks for the superior family Fritzl believed himself to be making, one sees that similar horror might occur on a tiny scale, for the same reasons.

It's strange how we know perfectly well that men such as Hitler or Stalin, Mao or Mugabe, can commit vast terrors and persuade others to commit them in the name of their grotesque vision of a "utopia", but cannot accept that Fritzl's own justification really may have run along similar lines. One totalitarian nightmare is too big to comprehend, and another one too little. Here be monsters. Stay away.

Was it merely that psychic sign that protected Fritzl so long from apprehension? One can understand, almost, that his wife was so controlled by him that she was rendered an unquestioning slave herself. But the neighbours? The authorities? It is easy to wonder, in retrospect, why no one ever became suspicious enough to investigate. Maybe it is simply because we prefer to believe that monsters don't exist any more, even though it's all too apparent that to be a monster, you also have to be a human.

d.orr@independent.co.uk