Maximilien Robespierre is an unpromising subject for a biographer, who has just five years to work with. Little is known about him until 1789, when he became one of the children of a Revolution that duly devoured him in 1794. Born at Arras in 1758 and educated in Paris, he practised law in his home town until politics called him to the capital. He never travelled further than Versailles, never owned a house, willingly sacrificed friends and associates to his political principles, and was so indifferent to sex, drink and bribes that he was called The Incorruptible.
He was famous for his green-tinted spectacles and dandyish clothes. Apart from this sartorial indulgence, he was as hard on himself and others as the most censorious puritan. The facts of his life are dull; his political career is alarming and unpleasant. In the absence of significant personal writings, he has to be approached from the outside and deductions made from his public pronouncements and political actions.
He was elected to most of the national parliaments established by the Revolution but his power-base was extra-parliamentary: the Jacobin Club. Like many others, he began as a constitutional monarchist who believed that the hated old régime could be modified in line with progressive Enlightenment ideas. But it soon became clear to him that the Revolution had many enemies within and without.
His moment came in July 1793 when he was elected to the Committee of Public Safety. He used his power to institute a reign of Terror which lasted from September until 28 July 1794 when, his jaw half shot away by a botched suicide attempt, his head was removed by Dr Guillotin's humanitarian killing machine.
Robespierre took as optimistic a view of human nature as Rousseau, with whom he identified closely. Human unhappiness is the fault of unjust societies which have allowed minorities with money and power to oppress majorities which have neither. Since human nature is intrinsically good, it should be enough to explain virtue to ordinary people for them to see it.
But while they might want goodness, they were clearly not ready for it. Le Peuple was ignorant and politicians as corrupt as the men of business. To educate them all for entry into the Republic of Virtue called for very firm handling.
Robespierre's political ideals - equality, freedom, justice, truth and reason - were shared by many Revolutionary leaders. But while a cynic like Mirabeau or a bruiser like Danton recognised that a residue of human baseness is ineradicable, the Incorruptible was as impervious to compromise as he was to humour. He ruthlessly suppressed groups he suspected of backsliding; first the Girondins, then the Hébertistes and finally Danton's party until, by May 1794, he stood alone.
The nobility of his ends justified his means. At first, legal proofs of guilt had been required against the Revolution's enemies, though no appeals could be lodged against courts which embodied the will of the people. Soon "moral proofs" were deemed sufficient: intuitive verdicts were delivered on the basis of the known character of the accused. The sentence was invariably death. About 2,600 men and women were guillotined between 1792 and Robespierre's fall in July 1794. Of those, 2,217 died in the last five months of his life. Yet Robespierre believed he was being cruel to be kind. The Terror he instituted was the thorny path to an ideal republic in which reason governs human affairs and power is exercised for the good of all.
Macaulay remarked that he would gladly prefer half an acre of Middlesex to a principality in Utopia. Ruth Scurr is of like mind. At times she loses patience with her subject, finding his character defective, his methods brutal, and his vision deluded. Yet her title accurately encapsulates her engaging fascination with this paradoxical man. "Purity" is a recognition of his unworldly innocence while "fatal" acknowledges the maxim that idealists, alas, make very bad revolutionaries.
But what makes this fine political biography of the birth of modern Terror particularly engrossing is its timing. Terror remains the dictator's policy of choice, extremists still use belief systems to unite their followers, and great powers export liberty by force to the lands of despots. Ruth Scurr's Robespierre reminds us that, scale apart, our modern crises are replays of a golden oldie.
David Coward's books include 'A History of French Literature' (Blackwell)