For most of his history Superman didn't have a no kill code, yes, he usually wouldn't do it because it was kids books, but he wasn't completely against it. At some point the writers decided that Superman should the the moral authority of DC and that screw up the character more than anything else, imo.
Wonder Woman and Captain America are morals and they are used as examples pf what we should aspire to be, and the fans accept when the story involve them killing, as long as it is show as justified and necessary. Superman kills a guy who is going to murder the whole planet and the only way to stop him is killing and half of the fanbase and the marjority of the critics go nuts, and will say that Superman should always find another way and that stories that force him to kill is not a good Superman story.
And the worse part is that half the people complaining only see Superman on the JL, the same people who say the character is OP bu their favorite can kick his ass.