Ye Olde Alignment system (Good/Evil Chaotic/Lawful, Gary Gygax) is an over simplified generality. A real character is more rich, and complex than a simple "always attacks" or "Fights only evil." I'm still not sure if I'm understanding the OQ correctly, but this is high Fantasy (It's got Orc, Tolkienesque High Fantasy) so ultimately yes, there should be at least one Evil faction, for the Good to fight against. Otherwise, there would be no reason for the Good to fight, and they can go back to Farming. That's what the Daedra are for, if you want to break it down to the most basic tropes.
However, I defy anyone to find a single 100% Good faction in Skyrim, or any of the other ES games. I'm not going to speak for Bethesda here, but I get the feeling they did that on purpose, especially here, where any given faction that starts off good eventually reveals the Dark Secret, or decides to issue an ultimatum that you may, or may not be comfortable with. Good requires evil, to contrast with, but Evil can exist for it's own sake. I see a lot of Black, and Grey morality here, which kind of lampshades how Fanaticism against evil often leads to Evil acts, such as the Vigilants of Stendarr declaring 0 tolerance, merciless in their war against Daedra, and Conjurers. (What's Stendarr the Aedra of again?) especially in such a combat oriented game. You have no option to feed the hungry, or heal the sick (Except by Death, "The only Cure") and you can kick down a Septim for the Beggars, but that loses it's novelty fairly quickly.