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.
I’m getting strong “blue steel” vibes.