In the bible it says:
You have heard that it was said to them of old: Thou shalt not kill. And whosoever shall kill shall be in danger of the judgment. But I say to you, that whosoever is angry with his brother, shall be in danger of the judgment. …And whosoever shall say, Thou Fool, shall be in danger of hell fire.
But if someone does something dangerous, and you want to highlight this to everyone (including the person who did it), is it ok to call them a fool or stupid? in order to highlight that the act in question was foolish or stupid?