SPOILER ALERTS AFTER THE IMAGE!
“Killing Zod was a big thing and Chris Nolan, originally, said there’s no way you can do this,” Goyer told the magazine. “That was a change–originally Zod got sucked into the Phantom Zone along with the others and I just felt it was unsatisfying and so did Zack. We started questioning—we talked to some of the people at DC Comics and said, ‘Do you think there is ever a way that Superman would kill someone?’ And at first they said ‘No way, no way,’ and we said, ‘but what if he didn’t have a choice?’ Originally Chris didn’t even want to let us try to write it and Zack and I said, ‘We think we can figure out a way that you’ll buy it.’"
“I guess for me... David and I had long talks about it and Chris and I talked long about it and it was like, ‘I really think we should kill Zod and I really think Superman should kill him,’” Snyder said. “And the why of it was, for me, that if it’s truly an origin story, his aversion to killing is unexplained. It’s just in his DNA. I felt like we needed him to do something, just like him putting on the glasses or going to The Daily Planet or any of the other things that you’re sort of seeing for the first time that you realize will then become his thing. I felt like, if we can find a way of making it impossible for him—like Kobayashi Maru, totally no way out—I felt like that could also make you go, ‘Okay, this is the why of him not killing ever again, right?’ He’s basically obliterated his entire people and his culture and he is responsible for it and he’s just like, ‘How could I kill ever again?’”
“I think that when you really put in stone the notion that he won’t kill, it erases an option in the viewer’s mind,” Snyder said.”
(Transcribed by ComicBook.com)