There can be no greater deficiency in the essence of the human being than forgetting what one has learned, increasing one's foolishness, and the other results listed above. It clouds are thinking, not just in the short run, while we are angry, but in fact effects our entire thought processes in the long run. With diminished intelligence and thinking ability, we have moved one notch further from being refined human beings and one notch closer to the animal world. Therefore, even if the anger was justified, it was still very detrimental to the human being. Fire burns you even if it was necessary and justified to put your hand in it.
Anger works on the intellectual dimension of the human being with the same mechanism.
Since Moshe got angry, he erred, forgetting the laws of forbidden vessels.