Hybris
Dreamer
If they're strict monotheists, and they worship the sun, then they would have to worship the sun exclusively. There can't be any other deities. The moon would be, at most, a non-deified subordinate to the sun. Perhaps its role would be similar to Mary's in Protestantism: she isn't divine, even if she is the mother of the Son of God, just a saint. But she isn't associated with negative things either.
If they're only as monotheistic as Catholics (God being the only recognized god but saints being prayable to, and Mary a recognized divine intercessor), there's room for the moon to be more deified. But that wouldn't necessarily make it the evil one.
The Moon might be the mirror of the Sun when the Night has comed, its way to still support its believers when he dived under the horizon. Pale reflect of the Sun, the Moon would be a bit like, you know, the little bit of "Yang in the Yin". And the "Yin in the Yang" when the Moon hide the Sun (Solar eclipses). Balanced