Is it legal to assassinate foreign leaders? In the US there is an executive order signed by Gerald Ford which says not, but any executive order can be repealed at the stroke of a pen. I am not aware of any US statute on the issue, and if there were one it would probably not survive a constitutional challenge. Congress does not have the right to repeal the President's inherent powers as Commander in Chief.
But what of the Common Law? The Common Law allows people to kill in self-defence. The meaning of the term is well-defined, and neither word means exactly what you might suppose. Self-defence incorporates some degree of pre-emption - you don't have to wait for the other side to strike first. It also does not have to be yourself you are defending, you can strike someone to prevent a crime against someone else.
There are, of course, limits. A reasonable person would have to suppose that a crime was imminent; your use of force has to be proportionate to the crime you are preventing; there has to be no other way of preventing the crime.
So, if for example we apply these criteria to Bashar Assad, what do we find?
Would a reasonable person suppose that he is about to commit murder? Well, yes, given his record. Murder is the family business. No reasonable person could suppose that this is the precise moment that he is about to stop.
Would assassinating him be a proportionate response? Again, clearly, yes. He has murdered many. Given the chance it is beyond reasonable doubt that he will murder many more.
Is there any other way of preventing him? I think not, though I would be delighted to hear of one. Most alternatives, such as diplomacy, gentle encouragement and threats, have already been tried. We could invade Syria, I suppose, but that runs a much greater risk of being disproportionate than does assassination.
It seems clear that under common law, assassinating Bashar Assad would be legal. The remaining question is this: would it help? Is it possible that it would make things worse, or at least no better. Perhaps someone else, at least as brutal as he, would step into the gap. Perhaps the country would disintegrate into civil war.
While these outcomes are possible, and in the case of some countries even likely, I think in Syria the most likely outcome is that the generals and stalinist hardmen would focus first of all on killing each other. There is no single obvious successor - one reason why Hafez Assad was able to impose his son's succession on a politburo full of men who thought they deserved the honour more.
It would probably be worth ensuring that Syria's democrats and dissidents are fully armed before triggering such internal baathist recriminations, but on the whole it is easy to conclude that the death of Bashar Assad would be something to be welcomed by all.