From Life after Death: A History of the Afterlife in Western Religion, p.179. Segal is a genius, and the usual idiosyncratic professor. I once heard him read an SBL paper from his lap top, no doubt finished on the plane (if indeed it was finished :-). He is hilarious and generally on the list for a party of any significance at a scholarly convention. Jewish scholar of all things Jewish, including the New Testament, professor at Columbia in New York.
"Though many think that dualism and monotheism are opposing phenomena, dualism actually seems to be a consequence of some difficulties with monotheism. From the perspective of ethics, monotheism is in opposition to polytheism, not to dualism. Once there is one god, he or she must be the author of all evil as well as good. Indeed, one might argue that dualism is not a stage on the way to monotheism so much as a stage beyond it, a strategic retreat from monotheism governed by the recognition that monotheism makes the explanation of evil problematic. In these dualisms, good will eventually conquer evil."
This passage appears in his treatment of Zoroastrianism.