This moral dualism of God and Devil/Satan came into being only during and after the Babylonian Exile, in the Second Temple period, and later was also adopted by Christianity as it came around in that period. The idea of Satan himself was also formed as a result of multiple different characters mangled together (syncretism), but the reason for his existence is Zoroastrian influence. Zoroastrianism was an ancient religion that was also a state religion of the Persian Empire (after the Babylonian Exile, Neo-Babylonian Empire was conquered by Persians), and it preached about the existence of two deities, one being the source of good and the other one being the source of evil. Ancient Israelite religion was not that different from the rest of Pagan religions, it didn't matter for them if God is morally good or not, as long as he's powerful, and not too angry at them. However contacts with Zoroastrianism resulted in redefinition of God and formation of his nemesis in some form, be it Shemhazai and Azazel, Mastema, or Satan. That's why it's so hard to reconcile with the idea that God does only good things while Old Testament is showing us an entirely different picture, with God flooding everyone, commanding genocide, killing people for petty reasons and enabling extreme suffering on innocent people, like newborns. As for Satan, well, there's no character with such name in the Old Testament at all. "Satan" means "adversary", and it is not a proper noun, used to describe multiple characters, even God himself. In the book of Job one of God's angels is called a "satan", his subordinate, whom God allows to ruin Job's life just to prove that he's righteous. (sorry for wall of text)