Superman appeals to us because of its somewhat mythic flavor. Every ancient civilization, regardless of technological level, has this story, of a superior species landing on earth, and eventually making earth his home.
the possibility of an ancient alien race living amongst us is a stuff of sci-fi, but it is definitely not far fetched. For example, Jesus Christ.
Christ is definitely an extra-terrestrial based on his own testimony and the attestation of his peers. He came into this world in a miraculous way---a sort of divine artificial insemination or what we call today as surrogate conception. in other civilizations, stories of similar births also populate popular culture. We have Samson and Hercules, two of the known strongest men who lived on this earth came thru "virgin births".
Of all the beings who were born this way, Christ was the only one who preached about the best way to get salvation. If you think about it, why would an alien race in this case, those who represent the "forces of Good", annihilate what they perceive to be of the opposite forces, or the "forces of evil.'?
Their argument really is, they did something "bad" and therefore, deserve to be annihilated. What if that something "bad" is perceived to be "good" in other worlds or realms? Who serves as an arbiter who determines that this act is totally bad and another act if totally good?
The argument about an act being evil and another act being good depends on a perspective. For example, killing one person. In all of humanity, killing is bad because it is morally or "intrinsically" evil to murder someone.
What if in other realms in this universe, murder is acceptable? Will we term that acceptance as "barbaric" and murder as "evil"? what is the basis of morality but a contrived set of values or norms established as forms of societal control?
What if going to the path of darkness is "good", and going to a realm of light is "evil"? Who then says that what we think of is the universally accepted norm and others are not?
IN the movie "Man of Steel", General Za's act of fighting Superman was not intrinsically evil. Like other generals, he was just doing what he was born for. In the case of Superman, when he killed Za, was he doing the right thing based on what his father wanted him to do?