While political assassinations carried out by the U.S. clearly violates our (oft violated) moral precepts as a country, that has not stopped us from attempting to pull them off over the last 60 years (quite a bit more in the last 10).  However, it looks like there is yet another argument against them: they don’t work.  The survival rates of terrorist groups where the leader was assassinated generally went up from 67% chance of survival to 83%.

I doubt that this will really do much to stop political leaders from attempting to assassinate leaders.  The point of an assassination is not really to increase security, but to show off power to the constituency.  Voters tend to remember high profile events like assassinations, not the slow-progress of real security actions.

Schneier on Security: The Effectiveness of Political Assassinations.