Actually, it is not a sin to be angry (I don't mean to sound rude if I am) as long as there is a just reason. Remember in John 2 when Jesus gets angry? Does that make him a sinner? This is one of the mistakes that the modern Bible translations have. It's also a reason to use the KJV.
John 5:22
But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.