If Gandalf claimed the Ring for himself, he had 50% chance to destroy Sauron 1v1. If Gandalf proved the victor, the result would have been for Sauron the same as the desctruction of the ring; for him it would have been destroyed, taken from him forever. But the Ring and all its works would have endured. It would have been the master in the end. Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained "righteous" but self-righteous. He would have continued to rule and order things for "good", and the benefit of his subjects according to his wisdom (which was and would have remained great). Thus while Sauron multiplied evil, he left "good" clearly distinguishable from it. Gandalf would have made good destestable and seem evil.

- From the Tolkien Letters.

And that is the problem with the Christian dualism of God and Satan. The only way good can exist, is if God is everything. The ring must be destroyed. We have to cast it into the fires of Mount Doom, which is inside ourselves so that neither side can hold the power. The Saga of the Lord of the Rings is in a sense, Tolkien's understanding of the spiritual journey. We are all our own personal Frodo and there's an adventure to be had.