This was a smart move on Lucas’ part. Though he professed to be making the film for adult science fiction fans like himself, he was also aware that there was a lot here to appeal to kids.
Lucas had become interested in science fiction as a kid, and he knew others did, too.
Many youngsters would attend this film, and a hero whose name included “killer” might send the wrong message. Even in those days, parents were becoming concerned about the influence TV and films had on children’s behavior, and a violent name for a character supposed to be heroic might arouse disapproval.
The name Starkiller might also have made Luke seem sinister, when he was supposed to be the opposite. It would have given the character a slightly dark persona that would have had to be overcome, an unnecessary complication in the plot. Though most would hate to admit it, even modern viewers have their prejudices.
We don’t make our heroes wear white hats anymore, but we don’t name them Starkiller, either.