1)
Why is an adulterer "Chaser Lev"?
1.
Rashi: He does not do so due to hunger.
2.
Malbim: The thief fills his Nefesh (satiates himself) via his theft. The adulterer ruins his Nefesh, and does not reduce his hunger - there is a small limb in man. If he satiates it, it is hungry! (Sukah 52b)
2)
What destroys his Nefesh?
1.
Rashi: Zenus.