As for it being obvious through his actions, we have to remember that in this particular version, we are still right at the beginning of Superman's story. It will become obvious through his actions that he is a good person. As for the things you cited from Man of Steel, I think those moments add a lot to his character. He is, at that point, still trying to figure out who he is, in a sense he is growing up. It's hard enough for any young person to grow up, but add to it that you are basically a god, that is bound to do a number on anyone's head.

Cowards teach their children to let others suffer and die in order to avoid risk to themselves. Selfish people teach their children to hide their gifts.

This is a very cyncial view of the mindset on the part of his parents. It wasn't about wanting or even letting people suffer. It was a parent's desire to protect his son from the negative consequences of displaying that kind of power to the world, especially before he is ready.

If I had a son with superman's power, I would teach him that he had an obligation to use those powers altruistically.

This is easy for any of us to say hypothetically, but if really placed in that situation, I think it would be much harder to do that. When your son has those kinds of abilities, there are numerous consequences to displaying them to the world: 1) People embrace him as a hero and showers him with love 2) He is ostracized because people are afraid him because of him being such an unknown quality 3) People attempt to exploit his abilities for their own gain 4) He makes himself a target for every person that wants to take him on because they have something to prove. There are many more, but the point is that there are numerous ways the world could react to someone like Superman and it is very much a mixed bag of good and back. To be able to handle that is a lot for such a young man who is still trying to figure out life. In then, Jonathan Kent doesn't want to risk what might happen to his son. It is too much of an unknown. Now, did he make the wrong decision? It is quite possible that he did. However, I think only someone taking a very cynical view of his actions would label it as selfishness or cowardice. Afterall, he wasn't doing it for himself. He was doing it for his son. He was doing it because he loved his son and was genuinely doing what he thought was right for him, even if it isn't what was best for the world. Every parent does this, it is just usually not on the same level as Superman's situation.

Also, in this version of the story, Jonathan Kent died when Clark was still young. Who knows what might have happened once he was a little older? Perhaps, when he felt he was ready, Jonathan might have set Clark down and imparted the lessons on altruism and charity to him at an age where he felt he could handle the responsibility and pressure involved in that. Ultimately, that is unknown as his life is cut short before that.