It's from the Empire interview. Here's the rest of the quote. I can see where you're getting the idea that this is meant to be the ultimate catalyst, but I don't take this as a primary reason (if that makes sense). He also cites the original ending being lackluster, puts a lot of story into Clark's parents (all of them) and there's the idea that adds to Zod's character, the "death by cop" quote.
“The ‘Why?’ of it for me was that if was truly an origin story, his aversion to killing is unexplained… I wanted to create a scenario where Superman, either he’s going to see [Metropolis' citizens] chopped in half, or he’s gotta do what he’s gotta do.”