IMHO, my moral obligations and responsibility to society is a personal question, not an Aikido one. We each have to decide where that line is and how we want to respond to it so we can live with ourselves. If I really beleived that someone will escalate and do more harm, and if I believed I really have the ability to respond, and if I don't have time to call the police, then I would have to live with knowing I tacitly let it go by. I would probably enter and attempt to take their balance and center and redirect them. First of course with good manners and a sense of humor. Only as a last resort, physical waza.