not killing has nothing to do with what you're saying OP....superman is viewed as a threat and not trusted by many now despite not killing (like that) because of how powerful he is, so him killing (like that) wouldnt change that...the only thing it would change is the number of people saying out loud 'see, i told you, we couldnt trust him'...currently, while many think it, only a few say it....
anyway, the reason he doesnt kill (like that) is because that's his character...he stands for truth, justice, believes there is right & wrong (no grey), that no matter what evil people do they can ultimately change and be good, etc., which includes only killing when he deems it the last and only resort....he is the ultimate good guy/hero, what people wish they could be/live up to (i'm not talking about powers neither), and the standard all other heroes are measured against....
side note: that also explains why when the shift in society towards more 'edgy' shyt started happening, people started disliking him as a character....and started saying shyt like 'he's not realistic'....