Well, it isn't as though Vladimir Putin has sworn off the use of political assassinations, Manc Skipper. Assassination is kind of his modus operandi.
I think the best argument against assassination is not a moral one. Quite the opposite actually. It is infinitely more moral to kill one wicked tyrant than thousands or tens of thousands of poor young men press-ganged into fighting his atrocity-laden war. Rather, the best argument against assassination is that if it is traced back to one of our governments, our national government has just committed a brazen act of war against a rival nuclear power which can be met with the most horrific reprisal.
I think an even more apt argument against assassination of any head of state that one considers (justly or unjustly and in this case justly) an enemy of monstrously criminal behaviour, is that it would set a precedent for such actions to become acceptable and, by that process, potentially customary.
Let the moralists sort out the morals, I don't want that can of worms to be opened alone on the practical repercussions that would serve in opening a new reality of international relationships, by which no Heads of State ever enjoy diplomatic immunity ever again.
Doesn't mean I'd be much perturbed if Volodya keeled over tomorrow from a heart attack or if some of his own offed him, but we shouldn't be involved in it.
Case in point being that it would have been great if the German conspirators of June 20 had succeeded in offing Hitler but the Allies offing him would only have found some acceptance if that had happened in the course of combat action (including an air raid) with a nation we (they) were at war with.
"We're" not at war with Russia.
I'm even of two minds over the US having turned over Saddam to a hostile Shia government that they (the US) installed, since there was no doubt that he'd be wasted by the very same. No matter how much he deserved it. A trial by an international court (better than Nuremburg at that) would have created more credibility of whatever judgment.
But yeah, even with large parts of the US administration and even its military sharing my discomfort (or I theirs) at the time, one can't always have it all.
Lastly I'll dwell, against my original intention, on the moral issue here wrt just one point. Offing minor echelons (no matter how little the "minor") may have become tolerable thru practice (Soleimani comes to mind), extending the practice to heads of state is something I don't want to see.
Let's face it, assassination is murder by the original linguistic (Arabic) origin of the word. Spanish (spoken where I live) has no other word for a murderer than "asesino". With a killer (not the distinction) being "matador".