I love Snyder's excuse...it's basically "After Superman commits genocide, he would never want to kill again."
Also, I'd like to point out that Snyder claims that Superman's aversion to killing is "in his DNA." This yet again shows that Snyder believes that Superman is who he is because of his Kryptonian birth, not anything done by the Kents. The man simply doesn't understand Superman.
He wasn't saying that was the case in the movie, he was saying that it was best to SHOW why he has this policy. Considering the number of people who complained about the telling rather than showing of things, this was a good approach, from a film making stand point. Just putting that out there first, the view of it from film making, before we go into whether or not it was right for the character.
I BELIEVE Snyder was saying that, in an origin movie: "if its truly an origin story, his aversion to killing is unexplained. Its just in his DNA."
That is, he was saying, without explaining it, then it is "just in his DNA."
Not that it's "genetic," but that it just "is." It's just "who he is," rather than there being a reasoning for it.
Now, I imagine that the contention here is that there IS a reason, and it the Kents. I agree, and I actually think it pretty much still is in the movie.
Clearly he has a solid moral grounding, and was raised as such. However, he was put in a situation that directly confronts his morals, and he was forced to make a choice. A choice he never wanted to ever have to make in the first place. That IS Superman, the idea that he would never want to make that choice.
Now that he HAS, as we saw in that moment, and his reaction, that aspect of his morality is even MORE cemented. Now he absolutely will NEVER kill. He will make sure he is NEVER in that position again. He had never encountered anyone the likes of Zod before.
In fact, he's never even had to really "fight" anyone. He's ONLY been saving lives*.
Similar to how, in the books, he saw killing Zod and his ilk as being the wrong thing to have done in the end.
Of course the books were able to give this aspect much more due, as they can far more easily spend a year on the aftermath.
I imagine, if they are smart, this will be a significant part of the sequel. It is a pertinent point and theme to touch on and carry through.
*Brings up the complaint others have had about him not showing a concern for people's safety in the film, yet it was pretty clear that that's ALL he was doing prior to being confronted with people he had to actually fight. Not to mention that one of the big aspects of the film was that he was essentially choosing humanity over what was left, and every potential greater future, of his race.