I thought the whole thing with Zod would make an excellent reason for Clark to develop a no kill code. I mean, from Clark's perspective that was a hell of a thing. He chose earth over Krypton, yeah, but Zod was still one of the only remaining members of Clark's race. Their people got that much closer to total extinction because Clark had to kill the guy. That's something I dont think any of us can really wrap our heads around. And for Clark, with his senses, having to watch all signs of life leave Zod's body must have made it much harder.
I'd much rather see Clark go through something like this and say "Yeah, I'm not doing that again. I'm the gods damn Superman and I'll find other options!" than him just saying "killing is icky and I want to be a good person so I'm not gonna do it!"
I mean, ultimately its kinda moot, because Clark kills when he has to, code or no code. But if he's gonna get a hair across his ass about the topic, having to kill Zod and drive his people closer to extinction is far better motivation than simple, all-purpose morals. No decent person *wants* to take a life. Clark saying he doesn't want to kill people is.....that's not a sign of nobility, it's a sign of common decency that most people share. It doesn't set him apart, and most people who don't want to take a life don't make a big oath about it. If Clark's gonna have such a clear-cut rule for himself, he needs a reason for that beyond "decent person." But like I said, he kills when he must anyway so whatever.