I would agree with you that someone would have to kill me before I would do anything like that. However an experience I had when I was a teenager gives me pause about how other people would react. I was with some friends and witnessed a man beating the living daylights out of another man. I was sure he was going to be killed. My friends ran and cowered behind a car. I grabbed the only weapon I had handy (a cast iron frying pan I had just purchased for my mother for her birthday) and swung out. I only caught the guy on the shoulder, but he grabbed his shoulder, turned and looked at me and must have seen something in my face. He ran. I have no idea if my friends would ever be able to NOT sit by and do nothing if something terrible were happening right before them.
Maybe I just have a few more life experiences than some people do, but I need all the information about a person before making a judgment. We are not all cut from the same courage cloth. I, personally, cannot fault someone for not stopping someone else if they fear for their own life.