Would DCEU's Superman cry about terrorists who kill children? I don't understand his moral compass at all. I don't get where this ultimate goodness comes from.
Where does anyone's ultimate goodness come from? Why does Steve Rogers have the morality that he does? Is it nurture or nature? If you believe it's nature, then you believe that people can't change and some people are born evil. So, if it's understood that generally ALL goodness comes from a combination of nature and nurture with nurture being the deciding factor, then that narrows down Superman's goodness as the result of a combination of his heredity and his upbringing.
Martha (and Jonathan) taught Clark, through his own adoption, to embrace that which is unknown or "other" with love and courage. Jonathan teaches Clark, through his own actions, that protecting life is important, but he also instills in Clark an understanding of how singular decisions can have wide-ranging consequences. He's told Clark stories, like the one Clark recalls on the mountaintop in BvS, about how a choice to save save his farm as a child ended up harming the farm and the horses of another's farm. When Clark doesn't fight back against his school bullies, Jonathan and Clark examine it from a moral lens:
Clark: I wanted to hit that kid. I wanted to hit him bad.
Jonathan: I know you did. I mean......part of me even wanted you to, but then what? Make you feel any better? You just have to decide what kind of man you want to grow up to be, Clark. Because whoever that man is, good character or bad, he's gonna change the world.
Again, emphasis is placed on how Clark's moral decisions will be weighed both in the micro and the macro contexts. When Jonathan sacrifices his life in MoS because he felt that if Clark had been exposed to save his one life -- one selfish decision from Clark -- it could doom a world and a son who weren't ready for the burdens of a savior hero, he was giving Clark the time to do exactly what he had just been arguing with Clark in the car before the storm hit:
Clark: I just wanna do something useful with my life.
Jonathan: So farming, feeding people. That's not useful?
Clark: I didn't say that. [...]
Jonathan: We're not your parents. But we've been doing the best we can. And we've been making this up as we go along, so maybe our best isn't good enough anymore.
This is the point where Clark begins his journey to receive more nurturing from his journey of self-discovery. Growing up, Jonathan had suggested to Clark that he was on Earth for a reason, and that one day he would have to discover what that reason was. That nurturing alone would fill you with a sense of purpose, and it was only enhanced when Clark learned more about humanity and himself from years as a wandering guardian angel culminating in meeting Jor-El who taught him about himself and his purpose:
Jor-El: Your mother and I believed Krypton lost something precious. The element of choice, of chance. What if a child dreamed of becoming something mother than what society had intended for him or her? What if a child aspired to something greater? You were the embodiment of that belief, Kal.[...] You're as much a child of Earth now as you are of Krypton. You can embody the best of both worlds. A dream your mother and I dedicated our lives to preserve. The people of Earth are different from us, it's true. But, ultimately, I believe that's a good thing. They won't necessarily make the same mistakes we did. Not if you guide them, Kal. Not if you give them hope. That's what this symbol means. The symbol of the house of El means hope. Embodied within that hope is the fundamental belief in the potential of every person to be a force for good. That's what you can bring them.
Notice the continued themes of nature versus nurture, which distills down to the idea that character comes from both and this applies to the individual and to society. Clark is the product of his Kryptonian genes and his upbringing on Earth. Believing in the power of choice and in nurture, Jor-El believes and encourages Clark to believe that others (humans) can also be nurtured (taught) to be better.
So, there you go. Clark's goodness comes from the interaction of his nature with his nurture, and that nurture instilled in him the belief in himself has a man with a purpose to create great change and to inspire hope but also a man who has to be prudent in his decision making. He is taught to love and to care for others with an open heart and altruistically in part because he was loved and cared for by the Kents (and accepted by Lois) in this way.
