Christopher Nolan didn't want Zod killed
Snyder: "In the original version of the script Zod just got zapped into the Phantom Zone. David [Goyer] and I had long talks about it, and Chris [Nolan] and I talked a lot about it. I was saying, ‘I really feel we should kill Zod and I feel that Superman should kill him.’ For me, the why of it was: if it’s truly an origin story, his aversion to killing is unexplained. It’s just in his DNA. I thought if we put him in an impossible situation, forced him into it, it would work. I felt like that could also make you go, ‘Okay, this is the why of him not killing ever again.’ He’s basically obliterated his entire people and his culture, and he is responsible for it and he is just like… ‘How could I kill ever again?’ Even though Zod says there’s no way this ends without it. ‘What are you going to do? Put me in jail? I don’t know what you’re going to do with me but I’m gonna just keep doing this until you stop me. I’m just a killing machine, especially now. I had a task before but you’ve robbed me of that too.’"
David S. Goyer: "So yes, originally Zod got sucked into The Phantom Zone with the others but I just felt it was unsatisfying and so did Zack. So we started talking to some of the people at DC Comics and asked, ‘Do you think there is ever a way that Superman would kill someone?’ And at first they said, ‘No way.’ ‘But what if he didn’t have a choice…?’ Originally Chris didn’t even want to let us try to write it but Zack and I said, ‘We think we can figure out a way that you’ll buy it.’ So I came up with this idea of the heat vision and these people about to die and I wrote the scene and gave it to Chris… and he said, ‘Okay, you convinced me.’ I’ve seen the film about four times now and everyone always gasps when it happens – they don’t see it coming – and I think it makes some people feel uncomfortable, whereas other people say ‘Right on!’ but that was the point. Hopefully what we have done with the end of this film is we’ve got the the mainstream audience, not the geek audience, to question it all. Hopefully we’ve redefined Superman."
Snyder: "I wanted to create a situation where Superman has gotta do what he’s gotta do or he is going to see these people get chopped in half. And I think Zod knows that. It’s almost like suicide in a way, it’s like death by cop. If Kal has the ability to kill him then that’s a noble way for him to die. It’s echoes the ‘A good death is its own reward’ concept in a movie, and if there were more adventures for Superman in the future, you now don’t know 100 percent what he’s gonna do. When you really put the concept that he won’t kill in stone and you really erase it as an option in the viewer’s mind, it doesn’t mean he doesn’t have a code.
"But again you’ll always have this thing in the back of your mind. This little thing of… ‘How far can you push him? If he sees Lois get hurt or he sees something like his mother get killed… you just made Superman really mad. A Superman that we know is capable of some really horrible stuff if he wants to do it. That’s the thing that’s cool about him I think, in some ways, the idea that he has the frailties of a human emotionally but you don’t wanna get that guy mad…"