Superman is a good guy of course killing will never be his first option. That's how it should be always written. Superman is not Wolverine/Punisher/ but he isn't Gandhi either.
The idea that somehow if Superman kills "captain unstoppable mass murderer" people will start to fear him is beyond ludicrous. I've been giving thought to that notion and find out it doesn't hold water. First off all the mere fact that a man with the power to toss mount everest into orbit is already frightening. I don't need to see him killing a known (and just as powerful) homicidal maniac to begin to be afraid of him. I mean using lethal force against mass murderers is not the only thing Superman would be doing all these years. He has been also saving people from burning buildings and all sort of natural disasters. Why would everyone start to fear him after he just saved the day one more time by slaying the dragon?
I think it should never get to the point in which Superman starts to casualy kill his oponents (no matter how powerful they are) it should always be a big deal for him to take a life.