In order to address this question, we need to select the few ingredients for our analysis, and limit these to only those necessary to the task. These are: Justice, justification and objective rigor.
The first, justice, needs to be strictly defined here, since in common usage it has an amorphous, malleable meaning more appropriate to emotional musing than to dispassionate reason. For the purpose at hand, justice means that quality that determines the appropriateness of consequences for specific actions or choices. Thus, justice has been served if the consequences that a person experiences are appropriate to whatever caused those consequences.
Justification means allowing an act that is presumed to be impermissible, because the circumstances negate that presumption. Homicide, for example, is presumed to be impermissible, but is justified in certain situations, such as self-defense.
Objective rigor is the quality by which the circumstances are wrung free of abstractions, symbolic and metaphysical twaddle and subjective agendas. The way the justice system attempts to achieve this is through adversarial proceedings subject to strict evidentiary standards. The matter must be capable of objective determination, free if ideoplogical flourish, and not merely be a metaphorical leap such as "killing the planet," or "hate speech." An examples of a permissible inquiry is whether someone is alive or dead.
Taking these principles and applying them to assassination, we can start with the presumption that killing a person is wrong, and therefore, to be permissible, must be justified. Justification comes from showing that the killling achieves a desired end; that is, according to the concept of objective rigor, preferable to the consequences of other courses of action. The key here is how objective rigor applies to the determination of whether the circumstances justify the act. This is what distinguishes this approach from "the ends justifies the means." The means must be necessary to an objectively proper outcome. In the case of judicial killing, i.e. the death penalty, a judicial proceeding determines whether the consequences of death are appropriate to the crime in question, and whether the accused is guilty of that crime. Another inquiry is whether or not the death of the accused is necessary to achiving justice. Western societies have increasingly found this not to be the case, with resultant decline in the use of the death penalty.
Justification is not limited to a retrospective analysis of past conduct. Self defense involves killling someone to prevent a future eventuality, i.e, harm to the person threatened. In such instances objective rigor must determine that the perceived threat was sufficient to overcome the presumption that killing has avoided an objectively undesirable outcome. The fear of injury must be objectively reasonable, and the method of defense must be as well.
In the case of Osama bin Laden, the principle of objective rigor must be applied to several considerations: How certain was it that he engaged in actions or made choices that made extra-judicial killing an appropriate consequence for those actins and choices; How necessary was killing to achieving an objectively appropriate result for those actions; How necessary was killing to prevent further objectively undesirable results from hos conduct. The answers to the first two questions probably do not provide confidence that killing was justified, although it is admitted that a certain emotional satisfaction might arise from it. The concept of objective rigor however does not admit such indulgences. If we consider the hypothetical wherein bin Laden had suffered a massive stroke, and was in an irreversible coma, the compulsion of bursting into his room and creasing his skull with a nine millimeter bullet wanes. There is problably sufficient objective reason to believe that bin Laden did a number of very evil things, and in other circumstances a more formal approach to establishing his culpability might have been warranted. Saddam Hussein, after all was executed solely on the basis of his past deeds, and this after a trial.
If justification is to be had, it derives from the concept of bin Laden as a threat, as one who demonstrated evil and murderous appetites and who was a great risk to indulge them again. He might have posed an immediate threat to his pursuers by, for example, harboring a bomb or alerting guards. In this case killing him on the spot would be justified, not because he was widely suspected of having done outragoues and evil things in the past, but as reasonably necessary to prevent him from doing them in the future. There are of course other benefits that can be argued from killing him in a raid; serving as a deterrent or warning to those who would follow in his footsteps, disrupting communicatins that would necessarily pass through him, and eliminating an incentive for disciples to commit mass murder for his supposed benefit while he was still alive.
Assassination can be justified if it is reasonably necessary to eliminate a severe threat. Allowing it in order to vindicate purely emotional needs opens up an entire world of pretextual and insular grievances that by their very nature can never serve as the basis for justice.