During World War II, Winston Churhill said that the British would summarily execute Hilter without a trial if he were captured. Some tyrants are so dangerous that their execution is justified, even without due process. If the only want to remove someone like Hitler is by assassination, it could be justified in international law.
It is a horrible truth no one wants to examine, but the fact is that nations can take actions that private citizens cannot. Furthermore, an ordinary person acting as an agent of a government can legally and morally take actions that he cannot as private citizen. That is why we do not prosecute soldiers. A government has a duty to defend its nation, and sometimes an assassination is the least bloody and destructive way to do that. The United States does not assassinate heads of state, but has the legal and moral right to do so.