I do not believe in the duality of good and evil, neither in the duality of God and Satan. The fresh milk we drink each morning if left untouched for the day will eventually spoil. But spoiled milk need not be thrown away for it can be turned into another form of food like cheese, yogurt, or cake. This is what is called 'Redemption'.
Evil comes from good.
It was good first before it became evil, but it can be good again. Likewise, Satan, the greatest of all angels was also good but eventually turned evil. He had a choice to be good again but chose otherwise and faced the judgement of his Maker. The Author of all things decides the future of things He creates. An artist, likewise, can decide to keep or destroy the artworks he creates.