Is it okay to kill leaders? Let's not quibble over state vs. business vs. terrorist vs. religious; let's just say they are leaders of some force and leave it at that. Is that oversimplifying? Perhaps. But I do it because otherwise we get into "well it's okay to kill terrorist leaders but not state leaders" and then next thing we have to define terrorist vs. freedom fighter vs. a leader who is making practical decisions for his state, possibly at the cost of others. It's a mess best avoided, so let's just straight to the generic: leaders.
Is it okay to kill them? I see two good ways to look at this. There are many bad ways, but I won't waste time with them.
One way is to say that it is the ideal action. Would we rather kill millions in a vast war or assassinate Hitler? I'd go with the second option. The idea behind this is that people are generally not all that harmful, not to say they're nice, but the average German won't invade Poland and shoot a lot of Russians without someone convincing him to do so. So shooting the Germans is really just going after symptoms, like ice packs for a fever when you really need anti-viral drugs (or actually anything except ice packs, you get a fever because the extra heat helps fight the disease). Instead go for the root problem: the leader. It is direct and much less harmful.
But is the leader the root problem? Perhaps not. Hitler did not magically become the leader of Germany; he was elected and there were many points at which he could have been removed from power. Leaders are no more powerful than their followers. In fact, the entire reason that tyrannicide would seem to work well is that leaders are not special, they die just like the rest of us, they have no ability beyond that of anyone else except for that which is given to them. In other words, the leader is just the focus of a larger societal problem. Killing the leader would not solve the problem, it could actually make it worse by triggering a power struggle which in the short term it would weaken the threat to the outside world, but when it ends it is likely that the remaining leader is the more dangerous one.
To twist the social problem back in favor of assassination, perhaps the lack of assassination is the social problem. When the legal processes are subverted and freedoms are overthrown, there are points at which the leader can be eliminated. It is not as if a state goes instantly from free and open to a fascist state of enslaved citizens; there are times when the leader is clearly bad but does not yet have total control. The fairy tale myth of the popular leader who rises to power and then shows his true colors, is just that, a myth. The failure to recognize and eliminate such a leader is the social problem and so elimination solves that problem.
Or does it? The problem with pre-WWII Germany was much more than just "they didn't shoot Hitler." There were others who would have seized power, though they'd have taken Germany on much different paths. Would he have gained power if not for the Communist rise in Russia spreading communism but also anti-communist sentiments which would then form the basis for many power struggles in the decades to come? What if the German economy was not weak? What if Europe wasn't plagued throughout history with anti-Semitic crusades? What if there was less racism and beliefs in the superiority of this or that ethnicity? Hitler wasn't born a violent anti-Semite, he became one and Nazi ideology, though not called it, existed before him.
Nevertheless, leaders are focal points and without them, otherwise dangerous forces may be dispersed or at least set back temporarily as a new leader grows.
I realize now that I never really directly addressed the idea of right or wrong, but instead only of practicality. However I feel I touched on it indirectly; when dealing with evil the main question is one of the practicality of various ways to control evil. If tyrannicide is practical, then it is moral. Well, it is if we could agree on who is an evil leader. We can't, so I just invalidated my previous statements and in fact made this entire post worthless. Damn.