I asked the question because the game leaves one with the impression that the sword is somehow important to them, and with that in mind, is it right to keep the sword when you know (or at least, have the feeling) that it belongs to them?
It is true that there is no proof of that, and don't get me wrong--I can't stand the Githyanki, I never give them the sword and have no qualms about killing them. But this is, after all, the Virtue mod, so I wondered if such actions deserve a virtue hit. On the other hand, if stealing isn't considered to be wrong, than there is no reason why refusing to give back the sword should be wrong, and like many people said the PC may have intentions that aren't entirely selfish.