He was the guy who brought QM to the US. Before then, it was a European thing. Also, although I'm not sure they went into it in much detail, he was published in nuclear physics. By the time of the MP, he was not a young buck contributor anymore (and physics was considered a young man's game at the time, might still be) but was very well known.
The moral code thing is more about the morality of building that bomb and the horror it might cause the world. I think Truman got it right, though: it was Truman who had to carry the moral burden of using the A-bombs, not Oppenheimer. Take Oppie, out, the bomb still gets made. But everyone will remember and debate Truman's decision.
I, do, agree with your take, though, that he was an asshole and didn't seem to care about many people other than himself.