Anger is a most serious character trait. The Arizal was more strict about the need to avoid anger than any other negative character trait. One must make every effort to overcome one's natural tendencies to get angry.
If a person is angry but contains his anger and doesn't say anything then it is easier to overcome than if he speaks out of anger. The Qol Sasson brings a comparison to hot water and fire to explain this. In order to cool down water that is hot, you pour cold water into it. This has the effect of cooling it down, silently, without any visual signs. If, however, it is a coal fire that you wish to extinguish, and the fire is, of course, very visible to the eye, pouring water on it causes much reaction in terms of noise and steam and the like, which emanate from it.
We learn from this, that when something is already visible to the eye and made known, the reaction is much greater. Our first endeavor must be to eradicate anger altogether. But if we find that it has crept up on us, we must keep it internalized and not speak words out of anger. This is because it is easier to be appeased if the anger was not made public, than to overcome anger that was accompanied by inflammatory words. But clearly, our ultimate goal must be to train ourselves not to feel anger at all.
(See Qol Sasson, ch. 35)
