In retrospect, after Empire came out, Owen's admission that he was "afraid of" Luke turning out like his father fit perfectly as one of the things in the original film foreshadowing the revelation that Luke's father is Vader. To me, that line really plays like Owen is aware that Luke's father turned out bad.
When watching Star Wars
back in '77, Owen's line "that's what i'm afraid of"
something horrible--possibly related to Vader. In the film--and when no one was thinking a sequel would be produced--Owen being fearful of some random Imperial officer would not make much sense, as they (the empire staffers) were faceless, but Vader was the standout--the mystery man with supernatural powers.
Who else would Owen refer to in that way?