"Monsters instead of people that symbolize moral weakness" is world-class nonsense : the monsters are often implicity, and in some case explicitly (eg, Fafnir, explicitly a human-like being turned into a dragon due to greed) the product of moral weaknesses. Other examples of monsters in classic mythology that are the fruit of humans mistakes, either directly (eg, a human's own failing turn him into a monster : Fafnir) or indirectly (eg, the failings of another human result in the creation of a monster : the Minotaur, Grendel). The link between the human and the monstrous is also highlighted by some of the most famous monsters of the late medieval and renaissance belief system : the werewolf (a human abandoning the civilized ways and becoming a slave to their bestial instincts) most notably. Vampires and even to some degree Witches (the Renaissance version) also fall into this scheme.
Then to compare that with Star Wars, when Star Wars explicitly goes out of its way to dehumanize Vader and paint him as exactly that : a monster. He, like the Ringwraiths (or the Orcs, for that matter) in Tolkien, have only the vaguest sense of humanity left to them ; they (Vader and Ringwraits), like Fafnir and werewolves, are humans turned into monsters by their own flaws. Obi-Wan even directly allude to this : "He's more machine than man". Vader is a cybernetic monster, or so we're lead to believe ; it's only after the Empire plot twist that we begin to see the leftover traces of humanity, that Luke will be able to use to redeem him in Jedi. The Ringwraiths, for their part, have no traces of humanity left, and can only be destroyed.