[1] Jesus said if you break one of my laws you break them all.And calling someone a fool says you have anger in your heart concerning that person.
[2] A sin is a sin in the eyes of GOD and do you want to be a man pleaser or GOD pleaser .And I dont believe being angry is a sin because jesus christ was angry at the merchant's in his fathers temple for they have turned his fathers temple into a market place.
So its how you handle that anger so why not give it up to GOD And it says in scripture whatever you say to one of mine you also say to me .So if you call someone a fool you are calling GOD one and I dont want to go there [DO YOU]