Yes, you’re right everything can cause serious harm or kill, but certain things are designed to harm or kill, or designed to look like something that could harm or kill.
Context is a big factor here. A person holding a 6 inch knife in a kitchen? No one is going to bat an eye. They’re probably using that knife to prep vegetables. Same knife, same person but they are walking around a market/playground/movie theatre? Suddenly it’s a very different situation and context.
Is that stranger with the knife safe? I don’t know, but I do know that I don’t have anything to defend myself aside from what is around me if they ARE NOT a safe person.
Thinking of your Superman example and an ubermensch having to think about everything, I think several comics and media that have explored aspects of this idea.
I remember reading that there’s a Spiderman story arc where it’s revealed that Peter Parker is holding back his “true” strength for nearly the entire time he’s been Spiderman. It’s only been his true strength of character that has made sure that he “pulls his punches” far enough back so as to not kill or harm the people he’s fighting or saving.
I also think about Robert Kirkman’s Invincible comic/animated series that explores how powerful people decide, either intentionally or accidentally, the fates of those around them, often with dramatic and violent conclusions. Invincible is the story of Superman if Clark Kent wasn’t raised by an American family in the mid-West and was instead raised for another more sinister reason.