Having already told us in EmunahSpeak: The Proteksia of Gentle Speech that gentle speech will protect one from anger, the Ramban then explains the importance of that piece of advice by explaining that anger is a most serious character flaw which causes one to sin.
While there are myriad reasons why this is so, Rabbi Yisroel Brog tells us that probably the most important of them is that when a person gets angry he loses his connection with reality, and having done so, even after he calms down he will still be disconnected from Hashem. Not only is he, as Chazal tell us, like one who is considered to have worshipped idols, and therefore as one who has no G-d, but he is also like one who has no self.
There are other character flaws, such as gaiva, that the Ramban could have warned us against.
Why dafka anger (ka'as)?
A Jew has to be able to contemplate, and ka'as disrupts the inner quietude upon which contemplation thrives. And it does it to such an extent that a Jew’s very essence atrophies.
In addition, ka'as is more than just an aveira. It’s a middah ra’ah that is the source of many of the worst aveiros such as gezel, loshon hora, sinas chinom, and machlokes to name but a few. The truth is that when Chazal said, if you never come to anger, you’ll never come to sin they were letting us hear loud and clear that ka'as is inherently a potential launching pad for every single aveira in the Torah.
The Rambam, for one, was listening because while it’s well known that he holds that people should avoid personality extremes by always seeking the middle way, the always was subject to one very big exception, the exception being ka'as, because when it comes to ka'as there simply is no middle way.
An individual who is trying to shake himself of ka'as should pull himself to the opposite extreme in which he will not get angry even in situations in which it would be justifiable for him to do so. And the result of this is as the Ramban tells us a little further along in the Iggeres, where he says once you have distanced yourself from ka'as, the quality of humility will enter your heart.
As can be seen, the further away we get from ka'as, which is the most serious character flaw, the closer we get to anavah (humility) which is the very best character trait one can posses. And the closer we get to humility the closer we get to Moshe Rabbeinu who was the most humble person that ever lived, which logically should have put him the furthest away from ka'as of any person that ever lived, and yet, Rabbi Dovid Miller reminds us that ka'as was the only point of weakness that gave Hashem cause to complain against him which resulted in Moshe being prevented from entering Eretz Yisroel.
Such is the power of ka'as, the most serious character flaw, that it could even affect Moshe Rabbeinu, the most humble person who ever lived.