Many people mistakenly believe that while God is totally good, Satan, or the Devil, is totally evil. They are polar opposites of each other.
This idea, however, is false. Satan, while being totally evil in a moral sense, is not totally evil in a metaphysical sense. Theologian Norm Geisler explains the distinction in his book If God, Why Evil?: A New Way to Think About the Question. Geisler writes:
The Bible speaks about Satan as “the evil one” (1 John 5:19) who is a liar by his very nature (John 8:44). Surely there is no good in Satan – is he not totally evil? Yes, he is completely evil in a moral sense, but not in a metaphysical sense. Just like fallen humans still have God’s image, even so Satan has the remnants of good that God gave to him as a created angel.
For example, Satan has good insofar as he is a creature of God, insofar as he has intelligence, and power, and free will. Of course, he uses all these God-given good powers to do evil; he is ever, always, irretrievably bent on evil. But this is only to say he is totally depraved morally, not that he is totally deprived of all creaturely good metaphysically.
God, on the other hand, is totally good, both metaphysically and morally. They are not opposites in a metaphysical sense. In fact, Satan could not even exist unless God created him. Evil is a corruption of good, a parasite. A personal agent who is totally and completely evil is, therefore, impossible.