Anger (Kaas)

Anger is a negative trait (midah ra'ah) rooted in the red bile [marah adumah], which descends from the kelipah opposite the gevuros called "shor."

In Tanya, it is explained that this trait stems from the element of fire in the nefesh habehamis. Just as fire constantly strives to rise upward, anger emerges when a person feels their honor has been injured and similarly desires to rise up like fire.

Severity of the Sin

The holy Zohar writes that through the fire of anger, a person ignites the fire of Gehinnom:

"Fortunate is one who guards a dwelling place for Shabbos in his heart, and does not allow the sadness of the spleen to approach, nor anger which is the fire of Gehinnom, about which it is said 'you shall not kindle fire in any of your dwellings on the Shabbos day.' And so it is certain that whoever becomes angry, it is as if they have kindled the fire of Gehinnom."

Furthermore, our Sages taught that whoever becomes angry is as if they worship avodah zarah. The reason for this is that during anger, emunah departs from them. For if they truly believed that this was from Hashem, they would not be angry at all. Even though a person who has free will may curse them, strike them, or damage their property - and is liable in both earthly and heavenly judgment for their poor choices - nevertheless, it was already decreed from Heaven upon the victim, and Hashem has many messengers.

Moreover, even in the very moment when someone strikes or curses them, the power of Hashem and the breath of His mouth that gives them life and sustains them is clothed within that person, as it is written "for Hashem told him to curse." And where did Hashem tell Shimei? Rather, this thought that fell into Shimei's heart and mind descended from Hashem, and the breath of His mouth that gives life to all their hosts gave life to Shimei's spirit when he spoke these words to David. For if the breath of His mouth were to depart from Shimei's spirit for even a moment, he would not be able to speak anything.

For this reason, the Beinoni, as soon as any hatred, jealousy, anger, resentment or similar emotions arise from his heart to his mind, God forbid, does not accept them at all in his mind and will. On the contrary, the mind rules and governs the spirit in his heart to do exactly the opposite - to conduct himself toward his fellow with the attribute of chesed and extra affection known to him, to endure from him to the furthest extreme and not to become angry, God forbid.

For Further Reading

Emunah Departed From Him, in the 'Chayei Rebbi' section of Kfar Chabad weekly 1899, page 52