I'm actually not sure if it is technically legal but with drone technology we are able to target and kill enemies without even being in the area. It may not be called an assassination, but I think our leaders would order an attack to take out a Dictator if they thought that it would save American lives.
Assassinations are essentially not protected by law. However, dependant on numerous variables, an assassination can be "allowed". If the assassination of an individual is essentially more beneficial for the greater good of the people, I believe the government will "turn the other cheek" and allow such a mission to be carried out.
Assassination of a dictator is a bad and illegal action under US and world laws. Assassinations of this type may lead to reprisal killings of prominent US persons. For example, terrorists often take credit for attacks that kill Americans and claim the attack took place as a reprisal killing. America needs to set a strong example of what is acceptable for nations to do and refrain from doing. While dictators may be evil and hurt the people living in their country, killing that dictator without a plan for a new regime will lead often lead to a civil war or prolonged civil unrest. While the US didn't assassinate the leaders of Egypt, our leaders must look at this nation's troubles as an example of what might happen in a destabilized nation.