Cultures with true monotheistic beliefs developed a god who was both good and evil.  As they struggled with the good and evil compulsions within them, so too, did they believe, that God also struggled.  This god  acted as sole proprieter of good and evil in the world and used them both judiciously.  However, civilizations who wanted to believe that God was a wholly good being found themselves with a paradox as they tried to explain the existance of evil in the comsos.

    A parallel to these beliefs was the belief in two gods of equal, but opposite power.  A good god and an evil god, if you will.  Early Indo-Iranian religion had two sets of these gods, the ahuras (good) and the daevas (evil).  The ahuras fought and defeated the daevas.  The leader of the ahuras became Ahura Mazda (the god of light) and the daevas were demoted to evil spirits ruled by the Lord of Darkness.
    Note:  The manner of which the daevas were made evil spirits follows a pattern reinforced by later beliefs.

    As the ever increasing problem between monotheism and the existance of a purely good god grew, polytheists developed expression through opposites in individual dieties who act beneath a wholly good god.  Beings with two personalities or twin gods.  The Iroquois had a legend about the daughter of the earth who gave birth to two sons.  The younger son, Flint, lives only to undo the work of his older brother.  As the older brother creates animals or crops or people, Flint throws up mountains and ridges to divide the tribes and undermine his work.        
    Note:  The existance of two gods opposing each other is the underlying theme of Christ and Satan.

    Evil aspects of a god are popularly associated with an underworld.  The Greeks and Romans believed in Pluto or Hades, the dark god of death and fertility, ruling a dreary underworld.  Even Ancient African beliefs note blackness and darkness as being associated with evil.  Dating even further back, chaos (a personification of darkness and void) has been linked with the existance of a Devil.  While the Bible states that before God made earth there was a dark void, an abyss, from which darkness wrapped darkness, we can see that in one sense, chaos is good.  It is the creative potency from which all was created.  We find such art and creativity intertwined with evil throughout historical and cultural interpretations.

    Early Christian beliefs showed God employing Satan's services as an evil spirit.  In the book of Job, God allows Satan to detroy all that Job has and, eventually, his own body.  This is seemingly done as an act of God, seeing that it was determined at a council in Heaven with God's consent.  
    In Samuel I, where Saul and David are interacting.  God sends evil spirits to corrupt Saul and throw a javeline at David's head.  Eventually, these spirits entice Saul to send David to his death at the hands of the Philistines.
    In Exodus, God hardens Pharaoh's heart repeatedly to prolong his misery.  As more punishments are enacted, God further hardens his heart and has him endure more.  It is an agent of God that slays the children of Egypt and the Pharaoh's own son.
    In early Christian writings, Satan is a word thrown about freely to describe anything that "obstructs" as that is the definition of the name from one translation.  He does not yet have an identity other than that of God's Messenger.

  We can see during the Romantic Age of Literature, an emerging character of Satan.  The Devil, as a figure of identifyable aspect exists in the Bible only in fragments strewn sporadically throughout the books, and only later assembled into unified concepts by writers and poets like Dante, Milton, and Blake.
    During the Romanic Age, Satan developed a personality and a more "human" aspect that he'd been lacking as God's messenger.  He was now a true adversary.  He was a being of pride and power, of vanity and deviousness unequalled by any other creature.
    Milton expanded upon the ideas of the Bible to give Satan a history where there used to be only function.
    Dante gave evil a more conceptual form by defining and describing the temptations that Satan represents.
    Blake's Marriage of Good and Evil almost paints Satan as a champion of freedom.  Though he cites Satan's self-righteousness as evil, he considered his rebellion good.  
    True interpretation opened up with the relaxing of the church's authority over the state.  While we developed colorful accents to his character, we also left the door open for people who chose to worship him as God.  Such is the choice and the freedom of choice that was instilled in every creature under the banner of "free will".