@SeventhSense; oh, I’m sure we do it; it’s unavoidable that governments will violate the law for the sake of pragmatism. But the point is that it still is a violation, and the laws are there at least in name, adding an important layer of complexity to those political ramifications you mention. There are consequences, and where there are consequences there is moderation. I could punch you in the face, but I would have to deal with the consequences of that action, and so I probably won’t. I only would if you really pissed me off; it remains a last resort for extreme cases, not a tool to use on a whim. The laws must be in place to force governments, or one global government, to reserve assassination for real threats and prevent them from using it as they please on their enemies.
I could be wrong, of course. Perhaps consequences caused by breach of law do not force moderation, as they must also force secrecy. The other side of the coin is that if, as the asker suggests, assassination were institutionalized, it would also be subject to regulation, and could perhaps be less chaotic and self-serving than it is today. I cannot quite picture what form this would take, however. It would have to be the tool of a worldwide organization, acting unitedly, or else it would simply be an extension of wars between nations, self-serving and often counter-productive. But if such an organization existed, truly transcending national boundaries, then these tyrants are effectively rebels against a global state. Assassinating violent rebels may be permissible, but when a government falls into the habit of assassinating those who rise up in arms against it, it risks acquiring the habit of assassinating those who rise up with words; political assassinations, which would doom democracy to failure. It would be impossibly, I thing, to encode assassination in law so as to prevent the possibility of this decline. So I doubt that it could ever be brought out into the political daylight as the asker suggests.
I apologize for the possible incoherence or inconsistency of this argument (or novelette). It is late, and I welcome any rebuttals, which I will address on the morrow. Night, all!