@failures art,
I don't know if it has been asked, but shouldn't it be hypothetical question rather than theoretical question? A theory is based on evidence, of which there is really none for extra terrestrials--no matter how likely their existence may be. A hypothetical question follows the form of "let's say" or "what if" or any one of the other similar qualifiers.
But anyway, "what if" discussions on the nature of extra terrestrials are generally insipid since they are often based on fictional depictions of aliens. Everyone has some sort of image of an alien based on movie or book depictions that were provided by human creativity, not actual reality. Thus, I fail to see how any of this has any thing to do with philosophy.
As to the last line "would the aliens be ethically justified in their actions because they are a superior race?"---this just seems like a hidden way to ask a racist question. Replace aliens with race or peoples and you generally have the situation of one group that is superior for some reason doing what they will because they are "superior" using that label as a means to rationalize unjust actions.
So to ask answer your question. No they would not be ethically justified. In the end, might does not make right.