Bernard Crick navigates the moral maze and ponders the murder of leaders who pose a threat to their subjects

As John Wilkes Booth leapt from the box on to the stage, having shot Abraham Lincoln, he shouted what he thought to be an appropriate Latin tag: " 000000000000 " - "that's the way with tyrants" or, more literally, "ever thus to tyrants". The phrase would have been associated by the well-educated in the audience with Marcus Brutus (who had been carrying on something of a family tradition), yet it was familiar to most of the Washington audience as the state motto of Virginia, framed in the days of rebellion against King George and the British. Booth was reviled as a mad villain by Unionists but glorified as a hero by many Confederates.

And now, in our own green and pleasant land, it has become a criminal offence to glorify terrorism. But how far will the new offence stretch? I hope not retrospectively. On the night of Bobby Kennedy's assassination in 1968 a good friend phoned me, her voice quivering with emotion: "Bernard, the killing of the two brothers is so terrible, so wrong; but we must not let go of the doctrine of tyrannicide. We must draw distinctions."

The late historian Irene Coltman spoke as if we were members of a small sacred (if secular) order preserving ancient truths about the origins and condition of political freedom. "When you write your intro to Machiavelli's Discourses you must remember that he praised the sons of Brutus." "Ah, yes.

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OK Irene, I will." The world of classical republicanism was very close to her. She and her husband, Roland Brown, then Attorney General of Tanzania, had been close to President Julius Nyere when he had instigated a translation of Shakespeare's Julius Caesar into Swahili, so that if he ever acted like Julius Caesar ... the message was clear.

Actually, I did better than recall the sons of Brutus in the Pelican edition of Machiavelli's Discourses . On Monday April 7, 1969 (this will save Special Branch searching), I broadcast on the BBC Third Programme a talk with the title "Should tyrants be killed?", subsequently printed in the good old Listener .

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My title was rhetorical. The plain answer in the tradition of Western political thought was "yes" - and I said so clearly. I did not have to rely only on the thinkers of Greece and Rome who honoured tyrannicides but denounced assassins. Many of them took for granted that their fellow citizens would understand the distinction without the need for formal definition - just as Tony Blair scorned quibblers in Parliament and said that "ordinary people understand what is meant by the glorification of terrorism". But to do better than that I turned to St Thomas Aquinas who, in the Summa , echoed Cicero's praise of tyrannicide, albeit on four strict conditions: 1) that the man to be killed had usurped power violently; 2) that he had broken the divine and the natural law and was a threat to the lives and morality of his subjects; 3) that there was no other remedy; 4) that his killing would lead to some better state of affairs. It must not be done for vengeance or for punishment - those matters were in God's hands.

Now, of course, in the modern world so much power is in the hands of party, military or state bureaucracy that the killing of one tyrant usually clears the way for another.

Condition 4 is difficult to apply. I would be hard put to name contemporary examples, even from Africa; and silence is more prudent (I am not George Galloway) even if, after all, the criminal charge would be glorifying tyrannicide, not causing it. If, that is, the courts reject - to the rage of Blair, and É«ºÐÖ±²¥ Secretaries, past and present, Charles Clarke, John Reid and my old pupil David Blunkett - synonyms. The late humourist A. P.

Herbert's Mr Justice Cocklecarrot might well rule that "tyrannicide" is "not terrorism" within the wording of the Act, and so I, if charged, would have to agree with such a shabby line of defence, perforce. For, of course, the Act will be interpreted by the courts and not by Blair's "ordinary people". Populism can go too far.

Now classical learning would have indeed seen Lincoln as a "dictator" - a consul holding absolute powers constitutionally for the time of the emergency - but not as a tyrant. Yet he might have been. Sometimes it is hard to judge which is the dramatic and moral problematic in Shakespeare's Julius Caesar .

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Much of our thinking about the relationship of ethics to politics was shaped by long or half-forgotten theological dispute and debate about the justification of killing tyrants. There was and is this ultimate justification of personal political violence. Not so long ago in historical time, Pope Gregory XIII (elected 1572) opined that it was a moral duty to assassinate Queen Elizabeth I of England, salvation guaranteed. Does that sound familiar? And there was also a Protestant theory of tyrannicide. A great man who invoked it in England has his statue right outside the public entrance to the House of Commons: Oliver Cromwell. Reformers modified St Thomas's first qualification because they were all too aware that even a legitimate ruler could turn tyrant, threaten the lives and even wage war against his own people. So, off with his head.

Thomas Hobbes, of course, would have none of this. The master of baroque prose burst out against young men "reading the books of policy... From the reading of such books men have undertaken to kill their kings, because the Greek and Latin writers, in their books and discourses of policy, make it lawful and laudable to do so - provided before they did it they called him a tyrant. For they say not regicide - that is killing of a king - but tyrannicide - that is, killing of a tyrant - is lawful."

But if Hobbes disliked the individual intentionality of the word, he sweepingly argued that "Leviathan" (whether a man or a corporate body) simply loses his authority if he threatens the individual lives of his subjects; and loyalty dissolves in battle when it is plain the cause is lost. So it is important to try to be precise about the quality of our horror at the death, for instance, of a Kennedy. Why should we all appear to have been more shocked by the death of two rich young men lusty in and for power (or by the latest murder blazoned in the tabloids) than by the premature death of millions by malnutrition and poverty or the failure to stamp out by force, when necessary, endemic local wars and even genocides - Rwanda, Bosnia and so on. Surely our horror at "mere" assassination or murder is found in the meaninglessness of the acts or in their irrelevance to the aims and effects intended - such as last July's bombings in London - rather than in the killings or bombings themselves.

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In Violence for Equality (1980), Ted Honderich went beyond Hobbes to claim that violent revolt was justifiable not just if the state killed its inhabitants arbitrarily, but when it was killing them slowly by deprivation and gross economic differentiation, as measured by huge life expectancy differences between ruling elites and subject populations. Perhaps the "for equality" was a rhetorical error. What he meant was degrees of inequality that lead to gross inequalities in life span and perinatal mortality - good measures of social justice indeed and ones that to any individual can appear as a fatality, yet are ones that any state should at least begin to remedy or face justifiable rebellion.

Some good souls think that they object to any kind of murder or killing. Absolute pacifists reject both capital punishment and war of any kind; and they presumably forsake even killing in self-defence, such as our common law allows, or rebellion against oppression. But political violence differs from individual self-defence or heroic self-mortification because it involves widespread human relationships and has widespread causes and consequences - and causes can be many and complicated and consequences often unexpected. In fact, there will always be some unexpected consequences with so many people directly or indirectly affected and involved.

Honderich has recently argued in After the Terror that although it is obvious to us all how we, in the US and the UK, were affected by 9/11 and the London bombings, we fail to recognise that we are all complicit in injustices of foreign and social policy that must, in common sense, have had something to do with the motivation of the killers. Our leaders, defending their failed policies in Israel/Palestine, Afghanistan and Iraq are either fools, knaves or hypocrites by denying any connection and traducing those who do. The terrorists choose methods unlikely to advance or justify their cause except to themselves and their sympathisers, and they do not define their aims as yet with clarity enough for political solutions.

In On Violence , Hannah Arendt argues that violence can be justified to remedy precise grievances but not world-changing abstractions such as "revolution" or "historical inevitability" - and today she might have added Osama bin Laden's holy war and President George W. Bush's War on Terror.

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Sir Bernard Crick is emeritus professor of politics at Birkbeck, University of London, and author of In Defence of Politics and George Orwell: A Life . This is an extract from and summary of his lecture on "Justifications of violence" to be given at Birkbeck on June 14.