Sha'arei Teshuvah 1:16: An angry person is prone to sin and open to damage.

Malbim: Man is compared to a small city, that a great king surrounds it to conquer it. The Yetzer, imagination and desire block him from conquering powers of the Nefesh under his authority. The Yetzer Tov fights for the city, and does not let the afflicters of the Nefesh to conquer the city. The elevated wall that stops the afflicter is the obstacle in front of the Ru'ach. Man's Yetzarim are mostly evil - images of pride, envy, vengeance... When the Ru'ach puts these images on the heart, then the enemy comes to the city, and war is kindled in the city. Then it is easy to be drawn after these images. Man has a power that rules to block the Ru'ach - "Kol Rucho Yotzi Kesil v'Chacham b'Achur Yeshabechenu" (29:11]. This is the wall that blocks the enemy from entering the city. One who has no block to his Ru'ach, and puts evil images on his heart, his Yetzer overpowers, and enemies of the Nefesh conquer the city, for there is no wall to block them.