Yes, I think that when a country has a ruthless dictator that assassination can then be justified. Sometimes, dictators are just too mean to their own people and then it is time to get rid of that particular dictator. When that time comes I believe it is just to use assassination for this measure.
Yes, assassination of a dictator can be justified on moral terms, because it can end human suffering. If a dictator hurts people, his removal can make society happier and more stable. If there is more to be gained by killing the one person who is killing others, it is tragic, but it is needed for the good of most people.
In my opinion, tere has been too much bloodshed under dictators, so killin them just makes more suffering and death. However, sometimes a death is needed, as incarceration may be impossible for whatever reason. While I strongly oppose Hussein's execution, it probably showed the Iraqi people that it is the end of a dictatorship. Symbolism was the only positive of Hussein's death.