Re: How Superman Resolved the Issue of Zod *MEGA SPOILER*
Superman's 'no killing' rule, the same rule used by characters such as Batman and Spider-Man, were included by the writers as a way to justify the limitations put in place by the Comics Code Authority. Now that it's gone, the rules should be relaxed, and in the cases of other characters it has been.
Superman shouldn't kill as a general rule because his powers give him an overwhelming advantage. He could kill every criminal on Earth but he doesn't because it's not right for him to do so.
I don't see, however, how this applies to a murderous supervillain who will not listen to reason and is directly threatening those around him.
If Superman let Zod kill those people, he would look like a fool. He would be directly responsible for their death. He would be a coward who couldn't bring himself to do what was right just because he doesn't feel comfortable doing it.
For me, Superman crying out in anguish over Zod's death is enough. It shows that he is not prepared to kill unless he absolutely has to. This makes Superman a stronger character rather than "I will not kill ever", because it shows he focuses on what is right rather than what makes him feel good.