Although sometimes it is necessary to assassinate a dictator, an assassination does not uphold the sovereignty in the international system. A sovereign state or country would pretty much be able to run their government as they see fit. So if someone comes in an assassinates a dictator, because they don't agree with the way they are governing, they are taking away their sovereignty.
One nation is pushing their will onto another nation when they assassinate another nations leader. This action is an act of war or could be considered such. There is no way this can be done peacefully regardless of what Hollywood would lead us to believe or what conspiracy theories on the internet try to say.