Colonel Gaddafi has promised to die in Libya, but not by his own hand. So long as he lives, the bloodshed will continue. Since Security Council Resolution 1973 calls for the use of "all necessary means” to protect the civilian population, it may soon appear that the means most necessary is the forcible removal of Gaddafi. Killing the worst man left in the world may seem excusable to some—he funded terrorism ranging from the IRA to Abu Nidal, probably ordered the destruction of Pan Am 103 and a UTA passenger plane, and on one occasion had 1,200 captives massacred in a prison yard—but we have scruples about the death penalty. Is it lawful to kill Gaddafi?

This question would have been answered resoundingly in the affirmative by the founders of international law. Grotius and Gentili held that in times of war, the usual immunity that protects sovereigns and commanders must be suspended, and they can lawfully be assassinated on the principle that “a man who is dead renews no war.” This was accepted in England: Coke, prosecuting Essex after his rebellion in 1601, explained that as an enemy leader he could have been put directly to the sword under martial law—the Queen had mercifully spared him so he could be put to death after being found guilty at a trial. This distinction could be important: Milton famously taunted the Presbyterian faction for opposing the execution of Charles I after years of “directing their artillery without blame or prohibition to the very place where they saw him stand.”

In time, war law (euphemistically called " International Humanitarian Law") became more nuanced: enemy leaders could be killed whenever "military necessity" required, although not by perfidious means such as spies or assassins or (in the case of Castro) exploding cigars. Clandestine CIA operatives, therefore, cannot terminate the Libyan leader with extreme prejudice. But in 2003, U.S. government lawyers, presumably with acquiescence from their British counterparts, approved "overt decapitation attempts" against Saddam: Two dozen cruise missiles and several 2,000 pound bombs hit his suspected hiding places. The U.S. publicly (if facilely) justified its attempts to kill the Iraqi head of state as a means of saving lives by bringing the war to a quick end, and there was (surprisingly) little or no protest. The Israeli Supreme Court, too, has been concerned to provide its forces with a limited "licence to kill" Hamas leaders in circumstances where "less harmful means"—i.e. arrest and trial—are unavailable.

If these precedents are legally correct, Gaddafi could be listed for liquidation. He is the enemy commander, whose lying promises of cease fires have been made for tactical advantage in a war where his orders are to show no mercy toward civilians in rebel-held towns. His attempts to put the lives of his people at risk, by urging them (women and children especially) to form “human shields” around high-value targets (himself the most valued of all) and to risk their lives by ordering them to march unarmed on rebel forces, engage Resolution 1973 and invite the "necessary means" permitted by the customary law of war.

However, care must be taken in relying on much criticised Israeli precedents or the 2003 advice of Bush lawyers whose opinions on torture and Guantanamo have been dishonourable and discredited. Human rights law with its reverence for life now trumps—or at least moderates—the harshness of traditional war law. Resolution 1973 has triggered the law relating to international conflict, and Gaddafi has lost his immunity by active involvement in the hostilities. But he can only be legally liquidated if this is the only possible way to save a significant number of civilian lives. If, as enemy commander, he possesses the power to end the war, he must be given an ultimatum to surrender or depart before the launch of any "decapitation attempts."

The days when dictators could exit the bloody stage with an amnesty in their back pocket and their Swiss bank accounts intact are long gone.

It has been suggested that Gaddafi be offered a "safe conduct." But the days when dictators could exit the bloody stage with an amnesty in their back pocket and their Swiss bank accounts intact are long gone. International law now invalidates amnesties for crimes against humanity, and as an ex-head of state Gaddafi would have no immunity if he were prosecuted by the International Criminal Court or by the Special Court for Sierra Leone, where he is an "unindicted co-conspirator" with Charles Taylor. He is wanted in France for ordering the UTA bombing (a civil case brought by relatives of the victims was rejected some years ago because of his "head of state" immunity), and the court of Scottish judges could be reconvened to consider new evidence from Libya's former Justice Minister that Gaddafi gave Al Megrahi the orders for the Lockerbie bombing. So if Gaddafi leaves Libya, it will be for that exquisite torture that takes the form of spending the rest of your life with your lawyers.

To the question of whether it is lawful to kill him, the answer must be "not yet." And when that time comes—which might be quite soon if his forces threaten bombardment or revenge in Benghazi—the fact that it is lawful does not necessarily mean that it is wise. Any targeted killing of Gaddafi would discomfort other dictators upon whom the West relies for support and would (in the words of Macbeth) "scorch the snake not kill it," since vicious LSE-groomed Saif would be waiting in the wings with his even more brutal siblings. But if the Gaddafis refuse an ultimatum to fold up their tent and depart, and the rebel forces (even with new U.S. arms) are on the brink of a bloody defeat, tyrannicide may become NATO's best—or only—option.

Geoffrey Robertson QC is a member of the U.N.'s Internal Justice Council and author of Crimes Against Humanity (The New Press).