Be angry, and sin not: the things you say in your hearts, be sorry for them upon your beds.
My question has to do with the first part of the verse: “Be angry, and sin not”. Is it possible to be angry and not sin? I can understand that you can be angry and not act on that anger (i.e., lash out), but what about not sinning in thought? Every time I get angry at something, I almost always end up sinning in thought (i.e., mentally insulting a person, lashing out in thoughts, etc.); to me it seems that anger and sinning (at least in thought) are indissociable; thoughts seem to appear as soon as I get angry.
He was referring to using your anger to override the temptation to join in the sinful activities. But note that this was before Jesus’ proclamation that what you do in your heart is paramount to doing it in body.
The very seed of sin is presumption and anger is even more tempting toward presumption than sexual lust, so your concern is a valid one as was Jesus’ comment.
One can behave as though angry so as to get a point across to an observer, but to actually “be angry” is to lust, void of consideration, toward an aim disregarding what might get broken by such thrust. Thus to actually feel anger is to tempt inconsideration of the whole, to temporarily ignore God. As such, it is not recommended unless you are a Secularist, Jewish, or Muslim. As a Christian, anger is turning away from your Lord (and makes you a little more stupid in the process).
767 In themselves passions are neither good nor evil. They are morally qualified only to the extent that they effectively engage reason and will. Passions are said to be voluntary, "either because they are commanded by the will or because the will does not place obstacles in their way."44 It belongs to the perfection of the moral or human good that the passions be governed by reason.45
1768 Strong feelings are not decisive for the morality or the holiness of persons; they are simply the inexhaustible reservoir of images and affections in which the moral life is expressed. Passions are morally good when they contribute to a good action, evil in the opposite case. The upright will orders the movements of the senses it appropriates to the good and to beatitude; an evil will succumbs to disordered passions and exacerbates them. Emotions and feelings can be taken up into the virtues or perverted by the vices.