[James Gunn] continued, “Yes, it’s about politics. But on another level it’s about morality. Do you never kill no matter what — which is what Superman believes — or do you have some balance, as Lois believes? It’s really about their relationship and the way different opinions on basic moral beliefs can tear two people apart.”
I disagree with Gunn here. Superman can and will kill, but only when it's absolutely necessary. It's a last resort usually reserved for the most evil and formidable villains. Superman would never kill Lex Luthor, for example, but Darkseid? Yes, he would.