The "this is man's world" thing is arguably handwaved. But it's a legitimate ethical question. Anyone who says "If I
had that kind of power, I'd
fix everything!" really should be able to come up with a bit of introspection as an answer to that. Would you? What is your moral standing to criticize some of the things you'd rid the world of? Who gave you the right to reshape things, as opposed to the power to do so?
We can always make exceptions to that, or well-what-about-Hitler questions, or whatever. But it's a fair question to ask yourself- just because you're sure you can answer it easily doesn't mean you don't need to have an answer. And Superman... doesn't seem to think he does have one.
Well, I don't think it's critical to Superman's character that he be especially intelligent, or good at philosophy. Maybe he just feels uncomfortable and exasperated trying to be policeman for nations, and rationalizes it as being immoral for him to do so.
Well if I was superman, I'd probably just settle for ripping the spines out through the chest of world leaders who started wars.
Or I guess he could drop war criminals off at the Hague, since it's pretty universally agreed-upon that those guys are worse than bank robbers.
But eh, maybe he does do that, I don't think I've ever read a superman issue.
There was one back in the very early days of the comic... hang on.
[looks up online, can't remember]
OK, he did exactly that in the second issue of Superman
(as in, back in the 1930s). And there was a little what-if about Superman getting pissed off and deciding to end the war by grabbing Hitler and Stalin and hauling them to stand trial in Geneva. Google 'superman hitler stalin' and look up the image results.
But since then, not so much.