If I may,
agreeing with BTNYC
Anger is an emotion, not a thought or sin at all. Like all things that belong to the material plane, it is in the use of it that one merits or sins. Christ's anger at the Jews for turning His Father's house into a den of thieves was used by Him to motivate Himself to construct a whip and use it to drive them out. If the fruit of ones action is bad or wrong, they have sinned using anger, but if the fruit of one's actions is good then they have merited. The severity of the fruit either way will help determine mortal sin. Also, free will must be involved which mitigates those times one just "flies off the handle".
I hope I've helped,
JoeZ