ספר ארחות צדיקים - השער השנים עשר - שער הכעס

Michael: I am pasting in here the first couple of lines of שער הכעס. I thought it might be easier to discuss with the text in front of us.

ספר ארחות צדיקים - השער השנים עשר - שער הכעס
הכעס היא מידה רעה, וכאשר הגרב מחולי הגוף - כן הכעס מחולי הנפש. ואמרו רבותינו (נדרים כב א): כל הכועס - כל מיני גיהנם שולטים בו, שנאמר (קהלת יא י): "והסר כעס מלבך והעבר רעה מבשרך", ואין רעה אלא גיהנם, שנאמר (משלי טז ד): "כל פעל ה' למענהו וגם רשע ליום רעה."

Let's go line by line. Rough translation: "Anger is a bad middah. And just as 'the itch' damages the body, so does anger damage the soul. And our sages say (נדרים כב א): "All who get angry--all types of Gihenom rule over him.' As it states (קהלת יא י): 'And remove anger from your heart, and remove evil from your flesh.' And there is no evil (as bad as) Gihenom, as it states (משלי טז ד): 'All the works of Hashem (he made) for His sake, even the wicked for the day of retribution.'"

Let us now take a look at the Gemara in Nedarim (22a) referenced above
"R. Samuel b. Nahmani said in the name of R. Jonathan: He who loses his temper is exposed to all the torments of Gehenna,13 for it is written, Therefore remove anger from thy heart,’ thus wilt thou put away evil from thy flesh.14 Now ‘evil’ can only mean Gehenna, as it is written, The Lord hath made all things for himself yea, even the wicked for the day of evil.15 Moreover, he is made to suffer from abdominal troubles, as it is written, But the Lord shall give thee there a trembling heart, and failing of eyes, and sorrow of mind.16 Now what causes failing eyes and a sorrowful mind? Abdominal troubles."
(13) V. p. 19, n. 6.
(14) Ecc. XI, 10.
(15) Prov. XVI, 4. This is understood to mean Gehenna.
(16) Deut. XXVIII, 65.

According to R' Shmuel bar Nahmani, quoted in the above Gemara, the angry person suffers both spiritually (i.e., "inheriting Gihenom") and physically. The physical suffering of the acutely angry person is well documented (i.e., higher incidence of heart attack, high blood pressure, ulcers, etc.). The spirtual suffering he will endure are the fires of Gihenom in the next world as a result of the evil he does while in a state of anger (as the OT will explain later on), and perhaps also for just getting angry in the first place, which (as we shall also see) is a denial of Hashem's Divine Providence.