Insanity is not necessarily a prerequisite for mass murder. Although killers such as the Russian cannibal Alexander Spesivtev and the serial killer Dennis Nilsen would reasonably be described as insane, there are some who kill their fellow human beings simply because they passionately believe themselves to be right. Sometimes these people - in the form of some suicide bombers - are brainwashed; but many times they are not, and they act out of a sense of religious or political fundamentalism, or indeed, out of obedience.

More ink than blood has been spilled about the motivations of the killers in the Nazi camp deaths and the Einsatzgruppen, but one observation that consistently emerges is that many of the perpetrators were not mad, and were in fact, in the words of Christopher Browning, 'ordinary men'. I have yet to read a serious thesis that advances the notion that all nineteen of the September 11 hijackers were insane. Although some members of the Provisional IRA were mad, the vast majority were not, and were able to commit mass murder nevertheless.

If you passionately believe you are right, and you feel you have no other method of obtaining your goal, then killing is a very logical thing to do. This is undoubtedly a normative form of human behaviour, as human beings have been killing each other for the "right reasons" for millennia. Many of us are repelled by the act of murder and, thankfully, we do not resort to it even if we believe the other side is wrong. But some do kill others to advance their interests, or to stymie those of others, especially if they believe that a greater threat is posed to society by not carrying out the killings.

Such an attitude was espoused by Heinrich Himmler in his speech made at Posen on October 4, 1943, in which he said:

Most of you here know what it means when 100 corpses lie next to each other, when 500 lie there or when 1,000 are lined up. To have endured this and at the same time to have remained a decent person [my italics] - with exceptions due to human weaknesses - had made us tough. This is an honour roll in our history which has never been and never will be put in writing, because we know how difficult it would be for us if we will had Jews as secret saboteurs, agitators and rabble rousers in every city, what with the bombings, with the burden and with the hardships of the war. If the Jews were still part of the German nation, we would most likely arrive now at the state we were at in 1916/17 [.] We have the moral right, we had the duty to our people to do it, to kill this people who would kill us [my italics].

For the purposes of the present argument, the key phrases in this passage are Himmler's assertions that it is possible to commit mass murder and to still be "decent" (in German, anständig, which also denotes respectability) and that there was a 'moral right' to carry out mass murder. Although Himmler was plainly wrong - the Jews did not present an eliminationist threat to German society - it is clear that the justification Himmler used to urge others to commit massacres was not born out of madness, but from a position of political fundamentalism and perceived preemptive self-defence.

Many members of the Nazi leadership were not insane and yet they appeared to be anständig. At the risk of sounding flippant, their attitude was one of "What happens in Auschwitz, stays in Auschwitz". Such a process, by which ordinary men are able to commit extraordinarily vile acts, has been labelled as 'doubling' by the American psychiatrist Robert Jay Lifton in his book The Nazi Doctors: Medical Killing and the Psychology of Genocide. Lifton's argument is that human beings, when placed in certain situations, need to be neither mad nor bad to commit acts that normally require a combinaton of just such attributes. Undoubtedly, it is the nature of those 'certain situations' that is crucial. It is easier to commit mass murder when there is a corporate intent to do so, and much harder to do so when acting alone.

What makes the case of Anders Breivik so troubling is that he committed mass murder on his own. For some commentators, such as Simon Jenkins in the Guardian, Sam Leith in the Evening Standard, and Boris Johnson in the Daily Telegraph, Breivik's actions are explained by insanity, and there is not much need to study Breivik's 'manifesto'. This, the argument runs, was the work of a lunatic who had built a puerile ideology to accommodate his psychopathy. In essence - the madness comes first, then the political justification, then the slaughter.

However, it is quite clear, at least to me, and Professor Andrew Silke on yeterday's Today programme, that Anders Breivik is not a madman. If you study Breivik's 2083 A European Declaration of Independence, it becomes apparent that Breivik is not insane. His thought processes are clear and rational. He appears to reasonably well socialised, and in control of his faculties. He even displays a sense of morality. His political arguments, although deeply wrong, are no more wrong-headed than many you might find in the comments boxes on the websites of many a newspaper, and, if anything, are far more sophisticated. Breivik comes across as an intelligent, thoughtful person, albeit obsessional with what he perceives to be the eliminationist threat of multiculturalism to Norwegian - and European - society. None of these attributes make him insane, and certainly not mad enough to be the psycho figure posited by Sam Leith and Boris Johnson.

The roots of Breivik's actions clearly lie in his politics, and when you read his 'manifesto', it is clear why he decided to act as he did. His argument runs thus: Multiculturalism, 'cultural Marxism' and immigration of Muslims is destroying our way of life. The people responsible for this are the ruling Labour Party. These people are traitors. I have tried to act politically, but that has yielded no reward, and little hope of doing so. Violence is the only solution. Therefore, kill the next generation of political Labour Party leaders. This is a necessary evil, but will save us from the greater murderousness of Islam in the long run.

And, in a brutally logical way, that is just what Breivik did. Those who he slaughtered, such as Hanne Kristine Fridtun and Tore Eikeland, were indeed future political luminaries, people who Breivik felt were traitors and would lead his country into the supposed darkness of Islamic domination. In his 'manifesto' Breivik suggests that his '"Justiciar Knights" would need to kill some 200,000 "category A and B traitors" in order to "break the historical 'Marxist vs. Conservative' cycle or we risk that the cultural Marxists will emerge as a dominating force again after 20-100 years". Breivik's thinking is, of course, twisted and evil. But it is not mad, and he is not unique.

Thanks to social media, Breivik was able to connect quickly - by his own reckoning - with thousands of likeminded souls on Facebook. Much of his 'manifesto' deals with building and maintaining such networks, and, of course, there is the suggestion that he met specific inviduals in London with similar intentions to his own. In this way, Breivik was able to feel that he was participating in a corporate enterprise, and, when combined with his political will, to have created a 'Lifton situation' in which murder became morally justifiable.

History shows us too often that the actions of certain individuals can - at the right or wrong moment - create vast consequences for society. The Norwegians are completely correct to insist that their ship of state should not deviate from its course. But we must learn how to stop future politically motivated massacres, and to do so, we cannot just dismiss Breivik as a madman and sail on blithely. Breivik's actions are not rooted in mental imbalance but in political belief, and we must study and negate his beliefs - and those who adhere to them - to stop future slaughters.