The Role of the Yetzer HaRa:

Rabbi Moshe Chaim Luzzatto (the Ramchal) in his work "The Way of G-d" describes the role of the Yetzer Hara is man's quest for perfection "Man is the creature created for the purpose of being drawn close to G-d. He is placed between perfection and deficiency, with the power to earn perfection. Man must earn this perfection, however, through his own free will... Man's inclinations are therefore balanced between good (Yetzer HaTov) and evil (Yetzer HaRa), and he is not compelled toward either of them. He has the power of choice and is able to choose either side knowingly and willingly..."

Ohr Somayach: The Ramchal explains that this balance existed prior to the sin of Adam.

After his sin, however, he became more inclined to listen to the wiles of his Yetzer Hara. His job is now two-fold: First, he must even the balance between the spiritual and the physical. Then, he can work to perfect his soul until his spiritual so overwhelms the physical that the physical becomes elevated to its level of perfection.

The Yetzer Hara is an inclination to "stray," but Man has the wherewithal to overcome it. The pull of the Yetzer Hara is the more powerful of the two inclinations, but is by no means impossible to conquer.

The Torah states:

"G-d said to Kayin, 'Why are you so furious? Why are you depressed? If you do good, will there not be special privilege? And if you do not do good, sin is crouching at the door. It lusts after you but you can dominate it.'"