Dictators are ruthless, power hungry people who will kill their own people for political gain. Just think what a world we would live in if Hitler, Hussein and Stalin were assassinated just as they gained power? Tens of millions of people would be alive today had those dictators been assassinated. Yes, a power vacuum would ensue after the assassination. However, bad men beget a bad end.
People can justify anything the choose to justify. People justify cheating on their spouse because they do not get attention at home. People justify lying to avoid conflict. People justify stealing because they want something yet they are unable or unwilling to pay for it. How is assassination of a dictator any different? It can be justified when a nation lives differently than another nation wants them to. Does that make it right? Some will say yes and some will say no but it will always be justified by some.
I believe assassinating a dictator is completely justified, if the dictator himself is viciously murdering his own people in highly brutal and horror acts. Sometimes these situations must happen in order to keep the innocent from living. If there is great evidence of his actions, where he refuses to surrender then he deserves to be put to death.