lovelylil.... Please note that Narkissos said that the Eden narrative (or any other OT text) does not identify the serpent with Satan and it does not even presume the existence of a "Satan" character; rather, the narrative portrays the serpent as an animal (see 3:1, 14), as the ancestor of all modern-day serpents (v. 15) in the same sense that Eve is the "mother of all living" (v. 20). Only an eisegetical reading of the passage (such as harmonistic with other later texts) would construe the serpent as "Satan". But acknowledging the original conception in Genesis does not preclude that later interpretations of the passage (such as the one in Revelation ch. 12, which however is equally dependent on OT and Canaanite/Israelite Chaoskampf traditions about Leviathan) existed, for that is exactly what is expected for such later texts (cf. the later interpretations of v. 15 as a Protevangelium or of the significance of the "fall of man"). The main point is to read each text on its own terms....
I would also like to note that only some streams of Second Temple Jewish thought have a "Satan" or "Belial" or "Mastema" adversary to God or God's purposes....in the Book of Watchers of 1 Enoch, for instance, there is no "Satan" character who is the source of evil....rather, evil results from a perversion of the natural order that results from angels (led not by "Satan" but by a group of prominent angels including Shemihazah and Asael) interferring with life on earth. Neither are these fallen angels active in promoting wickedness on earth today (as Satan is construed in Christianity) since they have been interred in Tartarus since before the Flood; instead, the forces of evil and uncleanness unleashed by their antediluvian activity still plague humanity even tho the original agents of evil have been punished.
There are other dimensions to dualism in early Judaism in addition to theological dualism (i.e. relating to the nature of God); there are also moral/ethical and apocalyptic dimensions which are closely related. Especially popular in Essene Judaism and early Christianity were the "two ways" moral instruction which construed the "way of light/life/God" and the "way of darkness/death/Belial" as diametrically opposed and which lead to different destinies (cf. the Manual of Discipline in the Dead Sea Scrolls, the Epistle of Enoch in 1 Enoch, the Didache, Barnabas, etc.). Since apocalypticism relates to the final judgment and punishment of one's deeds in the present world, dualism in moral conduct implies dualism of final destinies (e.g. Gehenna for the wicked and Paradise for the righteous). The Manual of Discipline, the Epistle of Enoch, and the "two ways" section of Barnabas each have apocalyptic connotations, whereas the ethical instruction in the "two ways" section of the Didache lacks such a perspective. See also the allusion to the "two ways" in the Sermon on the Mount in Matthew 7:13-14 which similarly combines both moral/ethical and apocalyptic (life vs. destruction) perspectives....