Back in 1986, Alan Moore wrote a famous story about Superman called Whatever Happened to the Man of Tomorrow? that became the swan song for the first 50 years of Supermans history. Terrible things happen to Superman over the course of the story seriously, you gotta read it and in the end, Superman has to make the ultimate sacrifice. Not suicide; Superman is the kind of guy who would give his life to save a kitten from a tree. No, Superman has to break his first rule and kill someone.
The person he kills is incredibly evil and responsible for untold terrifying actions. Much like in Man of Steel, he is immediately comforted by Lois: B-But you had to! You havent done anything wrong! Superman does not stand for that, though: Yes, I have. Nobody has the right to kill
Not you, not Superman
especially not Superman. Having superpowers doesnt give Superman the moral authority to decide who lives and who dies; if anything, it gives him less authority, since he has so much more absolute power to abuse absolutely. The crazy thing is that, in Man of Steel, his power is exactly what gives him the authority.