The Nezer HaKodesh explains the matter as follows: In truth, both are true of the yetzer hara. He is both an internal enemy and an external enemy. Man’s internal yetzer hara located in his flesh and bones stems from the corruption and perversion of his corporeal being. Additionally, man has an external yetzer hara that is an actual malach. He and his forces are well-known and are depicted by the passuk (Tehillim 12, 9): "סביב רשעים יתהלכון"—the wicked walk on every side. They are a constant bad influence aimed at leading man astray from the paths of life to the paths of death.
He analogizes the situation to a country that is attacked by an external enemy. In that situation, it is still easy to overcome the enemy. If, however, the enemy has also infiltrated the country, then it is much more difficult to defeat the enemy. In this manner, he explains the situation of Adam HaRishon prior to the cheit versus after the cheit:
Without a doubt, even before the cheit, Adam had freedom of choice and had a yetzer hara urging him to commit aveirot for the sake of reward or punishment. However, at that time, he only had an external yetzer hara but not an internal yetzer hara. For G-d created man just and upright; at that time, his material self was pure and untainted by the contamination of evil. Therefore, at that time, it was easy to escape from it.
After the cheit, however, man was also affected by the perversion of the internal yetzer hara. By eating from the Eitz HaDa’at, which contained the source of evil, the force of evil infected him and his body, and it became more material due to the corruption of evil. His garment of light (his body before the sin) was transformed into a garment of skin—namely, the skin of the contamination of the serpent that the serpent inserted in Chava, as explained in the Zohar in several places. As a consequence, man became surrounded by an internal enemy and an external one.
This explains the precise language employed by Rashi beautifully: "לא נתן בו יצר הרע עד אכלו מן העץ ונכנס בו יצר הרע". He specifies that the yetzer hara was not initially within him—“bo”—because the yetzer hara approached him only externally, as a malach disguised as the nachash. After he ate from the tree, however, “the yetzer hara entered him.” “Bo”—it entered his heart. Since then, man has been besieged both by an internal yetzer hara inside his body and a second, external yetzer hara, a malach.
