I don't know whether Israel did or did not assassinate the leader of the Hamas military wing, Mahmoud al-Mabhouh. But assuming for argument's sake that the Mossad made the hit, did it have the right to engage in this "extrajudicial assassination?"
Not all extrajudicial killings are unlawful. Every soldier who kills an enemy combatant engages in an extrajudicial killing, as does every policeman who shoots a fleeing felon. There are several complex legal questions involved in assessing these situations.
First, was the person who was killed a combatant, in relation to those killed him? If Israel killed Mabhouh, there can be absolutely no doubt that he was a combatant. He was actively participating in an ongoing war by Hamas against Israeli civilians. Indeed, it is likely that he was killed while on a military mission to Iran in order to secure unlawful, anti-personnel rockets that target Israeli civilians. Both the United States and Great Britain routinely killed such combatants during the Second World War, whether they were in uniform or not. Moreover, Hamas combatants deliberately remove their uniforms while engaged in combat.
So if the Israeli Air Force had killed Mabhouh while he was in Gaza, there would be absolutely no doubt that their action would be lawful. It does not violate international law to kill a combatant, regardless of where the combatant is found, whether he is awake or asleep and whether or not he is engaged in active combat at the moment of his demise.
But Mabhouh was not killed in Gaza. He was killed in Dubai. It is against the law of Dubai for an Israeli agent to kill a combatant against Israel while he is in Dubai. So the people who engaged in the killing presumptively violated the domestic law of Dubai, unless there is a defense to such a killing based on international principles regarding enemy combatants. It is unlikely that any defense would be available to an Israeli or someone working on behalf of Israel, since Dubai does not recognize Israel's right to kill enemy combatants on its territory.
If it could be proved that Israel was responsible for the hit--an extremely unlikely situation--then only Dubai could lawfully bring Israelis to trial. They would not be properly subjected to prosecution before an international tribunal. But what if a suspect was arrested in England, the United States or some other western country and Dubai sought his extradition? That would pose an interesting legal, diplomatic, political and moral dilemma. Traditional extradition treaties do not explicitly cover situations of this kind. This was not an ordinary murder. It was carried out as a matter of state policy as part of an ongoing war. A western democracy would certainly have the right and the power to refuse to extradite. But they might decide, for political or diplomatic reasons, to turn the person over to Dubai.
Turning now to the moral considerations, which might influence a decision whether to extradite, the situation is even murkier. The Goldstone report suggests that Israel cannot lawfully fight Hamas rockets by wholesale air attacks. Richard Goldstone, in his interviews, has suggested that Israel should protect itself from these unlawful attacks by more proportionate retail measures, such as commando raids and targeted killing of terrorists engaged in the firing of rockets. Well, there could be no better example of a proportionate, retail and focused attack on a combatant who was deeply involved in the rocket attacks on Israel, than the killing of Mahmoud al-Mabhouh. Not only was Mabhouh the commander in charge of Hamas' unlawful military actions at the time of his death, he was also personally responsible for the kidnapping and coldblooded murder of two Israeli soldiers several years earlier.
Obviously it would have been better if he could have been captured and subjected to judicial justice. But it was impossible to capture him, especially when he was in Dubai. If Israel was responsible for the killing, it had only two options: to let him go on his way and continue to endanger Israeli civilian lives by transferring unlawful anti-personnel weapons from Iran to Gaza, or to kill him. There was no third alternative. Given those two options, killing seems like the least tragic choice available.
I leave to others, more expert in these matters, whether if Israel ordered the killing, it was strategically the right thing, or whether they carried it off in an intelligent manner. But as to the legal and moral right to end the threat posed by this mass murderer, the least bad alternative would seem to be his extrajudicial killing.