The welcome end of Osama bin Laden at the hands of helicopter-borne American military commandos raises a number of issues.

Americans rejoiced at news of the end of this psychopathic mass murderer, and, privately, are probably relieved that he was not to be captured and extradited to Guantánamo. If bin Laden had been taken alive, we might be revisiting the controversy surrounding the Obama administration's failed efforts to try in a civilian federal court bin Laden's subordinate, Khalid Sheikh Mohammed -- the master planner behind 9/11.

But what, exactly, are the moral, legal or practical rules in going after terrorist leaders or the savage dictators of rogue regimes? We went into a foreign country to kill, not capture, bin Laden. Was that killing permissible since a firefight preceded it, or because he was a terrorist rather than a head of state?

Furor surrounded the waterboarding of Mohammed that purportedly resulted in valuable intelligence about future terrorist operations. But why was that considered immoral and illegal when we routinely act as judge, jury and executioner of suspected terrorists through predator drone attacks inside Pakistan?

Mohammed, a confessed killer, was one of just three detainees waterboarded. In contrast, we have executed from the air well over 1,500 suspected terrorists by Predators. President Obama has ordered four times as many drone attacks in the past two years as former President Bush did in eight. Are those killings more constitutionally suspect than Bush's treatment of the three terrorists at Guantánamo?

Last week, NATO warplanes deliberately targeted Moammar Gadhafi's family compound and residence in Tripoli, purportedly killing the dictator's youngest son, Saif. A surviving son, also named Saif, not long ago was a Western darling who bought a doctorate from the London School of Economics, and wined and dined Western intellectuals and oil executives. At what point do dictators' sons devolve from darlings to demons?

The United States had just days earlier sent two predator drones to Libya -- no doubt to help the British and French focus their attacks on the Gadhafi family. Are such targeted airborne assassinations the type of killings expressly forbidden by U.S. law? Or are they permissible on the grounds that enemy dictators are military commanders -- and their fortified homes are thus legitimate wartime targets?

Could we then legally, morally or practically drop a team in Tripoli to kill Gadhafi and his son in the manner that we killed bin Laden and his son? What are the rules that govern the killing of enemy leaders?

First, it seems OK to assassinate a terrorist kingpin either by air attack or commando raid. But legal and moral problems arise if he is captured, detained, waterboarded or tried in a military tribunal. A quick death seems to end almost all legal discussions and controversies.

Second, there is also no problem in assassinating a foreign dictator as long as the mission meets two criteria: We must be engaged in some sort of conventional battle with his forces, and we have to kill him through aerial bombing. For some reason, vaporization by a bomb seems to raise fewer ethical issues than execution by a sniper's bullet.

Third, targeted assassinations are better done under liberal presidents, who are more likely to be seen as humanitarians who only reluctantly order such killings. The Bush antiterrorism protocols -- tribunals, renditions, preventative detentions, Predator assassination missions, Guantánamo Bay -- were decried as illegal and immoral. Such furor vanished, however, when President Obama embraced or expanded them all. The effort to preemptively remove the mass-murdering Saddam Hussein to foster democracy in his absence was seen by many in the media, universities and legal community as morally wrong -- and yet preemptively bombing Gadhafi to foster democracy in his absence is now considered morally justified.

Fourth, success seems to end moral ambiguity in much the same way failure invites it. Had we gone into Pakistani territory and landed in the wrong compound, legal and ethical issues would have been raised. If we keep killing members of the Gadhafi family without hitting Gadhafi himself, at some point the denial of targeted assassination will seem empty. Targeted assassinations apparently have to work on the first or second attempt to be deemed moral and legal.

In recent years the United States has been in a number of undeclared wars against terrorists, insurgents and authoritarian dictators -- Mohamed Farrah Aidid, Saddam Hussein, Osama bin Laden, Slobodan Milosevic, Khalid Sheikh Mohammed, Manuel Noriega, Mullah Omar, Moammar Gadhafi, the Taliban, al-Qaida and others -- whom we sought to kill, capture or put on trial.

It is about time that we clarified the rules that determine their fates.