The answer
Before the foundation of the world, God created the angels with free will, and allowed a period of test. The angels were content being illuminated by the light of God, however, Satan rejected this light by wanting to be illuminated of himself. Hence, he wanted to put his throne above the throne of God, and so, like the cord of a lamp pulled out from the socket, he was cast into total darkness. Because God created Satan in the beginning, He can eradicate Satan, but God left Satan in order to give us the opportunity to experience free will; to choose between the way of light, and the way of darkness.
H.G. Bishop Moussa