Long before the Creator breathed life into the human clay, He decided to create the angel Satan, knowing this fellow would rebel against the Creator (omniscience).
The obvious solution to this rebellion would have been to uncreate (omnipotence) Satan, but the Creator decides to cast Satan into his own Kingdom, with immortality and various rival powers to those of the Creator.
Clearly the Creator was never reluctant to destroy that which displeased Him, in later eras, such as the Great Flood or Sodom and Gomorrah. Yet He spares Satan this fate.
Or short of uncreation, why not grant Divine Forgiveness to Satan for his transgressions, as Pat Condell asks (at 2:10 min), "... and then none of this nonsense would be necessary."
So the Great Enemy is established by the loving Creator.
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