incredibly common theme in many pre-Christian myth systems. the good/evil dualism in Christianity is the real aberration, despite its prominence.
one of the more credible hypotheses I've encountered about this phenomenon is that early Christianity adopted many of the positions of Gnostic Manichaeism [0] in order to absorb rival cults.
Norbert Wiener (of cybernetics fame) made a distinction between Manichaean devils and Augustinian devils-- essentially between evil as a force opposed to good vs evil as merely a lack of goodness.
Essentially the distinction is between mischief and error.
Christianity is far from alone in its belief in good and evil. The idea that there's a battle between good and evil goes back to Zoroastrianism, which predates Christianity by hundreds of years. The other Abrahamic religions, Judaism and Islam, also have conceptions of good and evil, though they might not be viewed as being involved in a battle.
one of the more credible hypotheses I've encountered about this phenomenon is that early Christianity adopted many of the positions of Gnostic Manichaeism [0] in order to absorb rival cults.
0- https://en.wikipedia.org/wiki/Manichaeism