According to biblical theology, Satan, Lucifer, or the devil was a high ranking angelic being, who rebelled against God, and was cast from His presence until a final judgement. He was not equal to God, and was a created being.
He was not created 'evil', but chose that path, implying free will for the angelic spirits, as well.
Many theories and speculations abound, regarding this historical enemy of God and humanity. Those who have encountered him, like those who have encountered God, may not know or understand the theology, but they know he is a dangerous enemy, intent only on lies and destruction.
Many questions arise. Did God know or not that, Lucifer would rebel?
Why God does not destroy the rebel instead of issuing warning to us?
Or is it an inherent quality of mind which we need to guard against?