Yes, the assasination of a dictator can be justified, although the act of assasination may still be condemnable. History have been witness to dictators (or maybe just Hitler) who have unjustly oppressed and murdered people, restricting them of the chance to live in freedom and peace of mind. And probably the most efficient way to allow ourselves ,if we happened to have lived under his reign, once again the chance to live in peace and freedom of mind is to assasinate the dictator himself.

Being a dictator, he/she have ultimate control of the economy and military arms of the country, which will give the citizens a grave disadvantage. Using other peaceful and less violent means to make the dictator stop will be futile, because the dictator can simply kill you at his discretion.

Besides, if we are to use the utilitarian judgment, the murder of a person who have caused the death or oppression of a thousand and will cause the death and suffering of another thousand soon will be of greater good to those who are still alive in his reign.

Of course, this judgment only under the assumption and prejudice that a dictator is someone who is as fierce as Hitler. Yet, if our dictator is someone who does not employ destructive means to enforce his reign (he may use force, but he does not do mass murders and the such), surely it could be more efficient if we just have to make a majority movement to change the government, not murder. Besides, he is a person, and we have ascribed the right to live to every person that exists. So, the act itself may still be condemned.

So, yes, a assasination of a dictator can be justified, but the act itself may or may not still be subject to condemnation.