I think Superman being so bent up about a mass murdering psychopath being executed, that he quits being Superman, is really dumb.
It's always killed KINGDOM COME for me.
I don't think Superman should be a killer, but if it was the Joker's life or 7 dozen innocent people being gunned down...I'd want Superman to make the adult decision. Sometimes being a hero is making a gut wrenching decision to save lives. Now simply executing the Joker after the crime, that's not really Superman at all...but I don't think he's going to lose any sleep over someone else being so upset that they kill the Joker.
If Zod is going to end all life on Earth, and there is no other way...you end the threat. There's literally no reason for a character like Superman to be put in situations where he can't "do the impossible" and save the world without killing Zod...but that's a different point.