If a dictator is killed, many deaths could be prevented. The killing would be for the greater good. Who wouldn't go back in time to kill Hitler? Thousands of lives would be spared. One death, in comparison, is trivial. These dictators do not just pose a threat internally - they are a threat to everyone. A dictator's assassination could even prevent a war.
Although most people assosciate assassinations with outside interference, this often isn't the case. During World War Two, Hitler's own generals plotted the Führer's downfall. These attempts were very close to being successful, but for a spy within the conspiracy. Even Joseph Stalin, the "Man of Steel", was not immune to traitors within his Polit Bureau. Julius Ceasar himself was killed by his assosciates and supposed best friend after declaring himself dictator for life.
When these people seize power, they are determined to hold on to it through fair means or foul. Their assassination may be the only way to change a country from dictatorship to democracy, particularly if an internal police force under the leader's rule has been upholding his or her will and preventing internal opposition.
A popular arguement against the assassination of a dictator is that there is an alternative to being them to justice. The leaders can be put on trial, and be held responsible for their crimes. The International Criminal Court...