Jesus himself called the Pharisees blind fools (Matthew 23:17). 1 This suggests that he is speaking in a categorical manner for the purpose of emphasis (see Luke 14:26) and not prohibiting the use of the word in an appropriate manner at an appropriate time.
22 But I say to you that everyone who is angry with his brotherwill be liable to judgment; whoever insultshis brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the hellof fire.