The biggest reason why assasination is frowned upon is that a decision not to frown on assasination would have to come from political leaders - the very people who would be targetted by assasinations. ‘Hey, sure, it’s been bad to shoot at me before, but now go for it buddy! Have at me!’ Although there are other points to be made, I think that that is really by far the largest reason assasination is frowned upon.
There’s also the practical problem of difficulty; assasinating Saddam Hussein, for example, would not be easy because he has lots of guards, various hardened locations, and other defenses. A democratic leader would be easier to target, since he has to interact with people more often, but that argues against assasination since the less bad countries are more vulnerable to it.
A further problem is that assasinating a leader doesn’t mean the country’s policy will change, in fact it might harden their resistance. If there’s a good chain of succession, or if someone is able to take control afterwards, they very well may continue or worsen the policies of their predecessor. If someone were to assasinate Bush, do you think that Cheney would turn around and reverse position, or do you think that he would continue doing the same thing, probably more adamtly since he wouldn’t want to ‘dishonor the memory of the Late President Bush’. And look at Iraq; we may very well have taken out Hussein on the openeing day of the war, but they aren’t doing anything differently than what we expected.
A related problem is instability; assasinating leaders can result in a civil war or other mess, and the replacement may not be in firm control of the country. This is bad; it might mean your new leader wants to do what you were hoping for, but can’t because he doesn’t have the loyalty of the people who’d actually do it. You might now have a civil war that starts spreading to nearby countries and destabalising the whole area. And it should be ovious that it’s not a good idea to try assasination against nuclear powers, since chaotic ICBM-armed factions are worse than pretty much anything going on now.
So, asside from the big reason, assasination is hard to pull off, probably worse for the better nations, not neccesarily effective in achieving a given goal, and quite risky even if it does succeed, especially with WOMD around. Does that mean that it’s a Bad Thing overall? That’s a question for GD, not GQ.