Sotah 4b #1: If one has haughtiness, it is as if he serves idols. This is like "v'Lo Savi So'evah El Beisecha" (Devarim 7:26).

Sotah 4b #2: It is as if he denies Hashem - "your heart will rise, and you will forget Hashem."

Sotah 4b #3: It is as if he had Bi'ah with all the Arayos. This is like "Kol ha'So'evos ha'El" (Vayikra 18:27).

Sotah 4b #4: It is as if he built a Bamah (private Mizbe'ach, when these are forbidden) - "Chidlu Lachem Min ha'Adam Asher Neshamah b'Apo Ki va'Meh Nechshav" (Yeshayah 2:22). We read "ba'Meh" like Bamah.

Sha'arei Teshuvah (1:27): Aside from causing sins, the Midah [of pride] itself is an abomination. A haughty person is handed over to his Yetzer. Hashem does not help him, for he is To'avas Hashem.

Malbim: Hashem does not punish for bad thoughts and Midos as long as they were not carried out. Also for action He does not punish immediately via special Hashgachah, rather, He hides His face from the sinner and leaves him to occurrences of the time. However, haughtiness is different. He thinks that he is at a very high level, so he breaches with all powers of the Nefesh to consider his heart like Lev Elokim. Via this, he does not heed the words of Torah and teachers. He breaches the boundaries of Musar.

Lev Eliyahu (Bereishis p.125): Even if he does not show his pride, the leaning towards pride in an abomination.

Sotah 5a: Anyone who has Bi'ah with a married woman, even if he gives Tzedakah discreetly, about which it says "a discreet gift subdues anger", he will not be exempt from punishment in Gehinom.

Rashi: The payment for his haughty heart will come to him from hand to hand.

R. Yonah: Yad is Chelek. This is like measure for measure. When Hashem pays for people's deeds, a haughty person will not be cleansed.

Malbim: He will not be punished via an intermediary, and he will not be cleansed from punishment. Punishment via nature, sometimes a Rasha persists a long time in his evil, for Hashem hopes that he will repent. There is no hope for a Gevah Lev to improve his way.