A Voice from the Past - Gregory the Great

But on the other head the passionate, in that they are swept on into frenzy of mind by the impulse of anger, break up the calm of quietness, and so throw into confusion the life of those who are put under them. For, when rage drives them headlong, they know not what they do in their anger, they know not what in their anger they suffer from themselves. But sometimes, what is more serious, they think the goad of their anger to be the zeal of righteousness. And, when vice is believed to be virtue, guilt is piled up without fear.

Gregory the Great, 540-604 AD, Pastoral Rule, Part III, Chapter XVI, (translated by Rev. James Barmby, The Nicene and Post-Nicene Fathers, Second Series, Volume XII,  Philip Schaff and Henry Wace, T & T Clark and Wm. B. Eerdmans Publishing, 1997, pp. 39,40)

How might we mistake anger for zeal? How can we avoid it?