Normally, someone is called a fool in order to show the contempt that one has for them. As such, it is a symptom of unrighteous anger and is inappropriate. In ancient times, a person’s name spoke to the essence of who they were, and thus to call someone fool
is to show complete disregard for them as a person. 1
22 Mais moi, je vous dis que quiconque se met en colère contre son frère est passible de jugement; que celui qui dira à son frère: Raca! mérite d'être puni par le sanhédrin; et que celui qui lui dira: Insensé! mérite d'être puni par le feu de la géhenne.