First, assassinations save lives. Full out military action costs lives on both sides, and costs civilian lives. If the assassination of one man saves the lives of many thousands, especially if that leader is the aggressor or is corrupt, then its justified.
Second, terrorist attack civilians, and therefore are outside of the Just War doctrine. The only effective means of combating terrorists is through statistic military strikes and assassinations.
Third, this would preserve resources for the military, keep us out of foreign nations, and allows us to focus on our own people without sacrificing their safety.
I don't think it is right to simply assassinate anyone dictator or not. There are other ways to about fighting a dictator. To decide to go ahead and assassinate them is simply stooping to the same level as the dictator. Peaceful means should be used in trying to remvoe them from power.
Assasins of terrist leaders is not approprate military action just as one would not want one to assassinate any leader. The genre of the government does not matter. It is the matter of the leader being the head of a country that makes assasins inappropriate military action. Assasinations are wrong.