Speaking at the BAFTA and BFI Screenwriters’ Lecture (via Digital Spy), Goyer explained why he had no problem turning Superman into a killer in Man of Steel:
“We were pretty sure that was going to be controversial. It’s not like we were deluding ourselves, and we weren’t just doing it to be cool. We felt, in the case of Zod, we wanted to put the character in an impossible situation and make an impossible choice.
This is one area, and I’ve written comic books as well and this is where I disagree with some of my fellow comic book writers – ‘Superman doesn’t kill’. It’s a rule that exists outside of the narrative and I just don’t believe in rules like that. I believe when you’re writing film or television, you can’t rely on a crutch or rule that exists outside of the narrative of the film.”
Goyer went on to defend their narrative choice by adding that they didn’t want to give Superman an easy out:
“So the situation was, Zod says ‘I’m not going to stop until you kill me or I kill you.’ The reality is no prison on the planet could hold him and in our film Superman can’t fly to the moon, and we didn’t want to come up with that crutch.”
Furthermore, Goyer fully expects to explore the repercussions of that decision in further films, like 2015’s Batman vs. Superman:
“Also our movie was in a way Superman Begins, he’s not really Superman until the end of the film. We wanted him to have had that experience of having taken a life and carry that through onto the next films. Because he’s Superman and because people idolise him he will have to hold himself to a higher standard.”