First of all,
Nolan disagreed at first. But then Goyer insisted to include the twist.
"Man of Steel director Zack Snyder and writer David Goyer revealed on The Empire Film Podcast that producer/comic book movie Godfather Christopher Nolan initially did not approve of the controversial ending where Superman kills General Zod.
“Killing Zod was a big thing,” Goyer said. “And that was something that Chris Nolan originally said, ‘There’s no way you can do this’.... Originally, Zod got sucked into the Phantom Zone along with the others.”
When that ending proved to be unsatisfying, Goyer had to convince others to go with the twist. “We talked to some of the people at DC Comics and said, ‘Do you think there’s ever a way that Superman would kill someone?’ At first they said, ‘No way. No way.’ We said, ‘But what if he didn’t have a choice?’ Originally, Chris didn’t even want to let us try to write it. Zack and I said, ‘We think we can figure out a way that you’ll buy it.’ I came up with this idea of the heat vision and these people about to die. I wrote the scene and I gave it to Chris and he said, ‘OK, you convinced me. I buy it.’”
Thoughts? I wasn't as shocked about it as many others seem to be. But for some, that was big.