Anger can be either a good or a bad thing

"You note that it says, “Then Nyagrodha grew extremely angry.” Under ordinary circumstance if one displays great anger toward a patron deity, one’s life in this present existence will be cut off and one will thereafter be born in one of the evil paths of existence. Nevertheless, this rich man, Nyagrodha, displayed extreme anger toward a patron deity and spoke to it in abusive language, and yet his great wish was fulfilled and he was granted a worthy son. From this you should understand that anger can be either a good or a bad thing." -- On Reprimanding Hachiman