Quoting from their 'Reasoning From the Scriptures Book':
"All of Jehovah's works are perfect; he is not the author of unrighteousness; so he did not create anyone wicked. (Deut. 32:4, Ps. 5:4) The one who became Satan was originally a perfect spirit son of God. When saying that the Devil "did not stand fast in the truth," Jesus indicated that at one time that one was "in the truth." (John 8:44) But, as is true of all of God's intelligent creatures, this spirit son was endowed with free will. He abused his freedom of choice, allowed feelings of self-importance to develop in his heart, began to crave worship that belonged only to God, and so enticed Adam and Eve to listen to him rather than obey God. Thus by his course of action he made himself Satan, which means "adversary." -- Jas. 1:14, 15