It's a reasonable belief and fear that person will carry out the act. You are probably misinterpreting the reasonable part, especially when people mask their reaction to try to get away or out of shock / instinct. The person making the threats probably gets enjoyment from getting a rise out of random strangers. Also, returning to the original context, there are too many people on the street that have severe mental health crises going on and are freely roaming some cities. They don't necessarily need to go to be convicted of a crime as a solution.