Iago: While I would argue that Iago is the epitome of evil and that as such he is an integral part of humanity, I don't believe that humans such as Iago truly exist. To me, he represents pure evil; Shakespeare makes a point of connecting Iago to the serpent who ruins Adam and Eve's paradise in the Garden of Eden. Iago is Shakespeare's only villain who seems to have no redeeming qualities. The extent of his malevolence overshadows any justification for his actions against Othello, Desdemona, Cassio, and even his own wife.
Desdemona: Desdemona is partly responsible for her fate. She lies to Othello about the handkerchief. While he most likely would have been upset about her losing the handkerchief, Iago would not have been able to frame Desdemona if she had simply told Othello that she had misplaced it. Similarly, Othello begins acting out of character--he becomes a different man from the gentle, controlled, Renaissance man whom she observed and fell in love with in Venice. This should have been a hint to her that she needed seek out who or what had changed her husband. Even at the play's end, she is too passive and goes too easily to her death.
Though Iago, Shakespeare shows us how individuals can act to other's detriment or ultimately to their destruction, in pursuing their own ends regardless if such actions cause others harm. Certainly one's actions have repercussions on others, and one would do well to follow the tenets of the Golden Rule, as actions have a tendency to come back to the initiator. However, throughout all time, there have been manipulators and schemers who pursue goals while wilfully harming others. What Shakespeare does in Iago is make refereneces to the devil within the play, and as a representation of Satan incarnate he suggests that such conduct is not only wrong, but sadly timeless.
It seem like there are two separate issues at hand in the question. The first would be about Iago's epitome of evil and how intrinsic he is to the notion of consciousness. Certainly, there is a universal aspect to Iago, as he does represent the depth to which humans can sink. The calculating and scheming ends to which Iago goes proves that his actions are not that different than other humans, whose actions are ones that lurk in shadows and wait for appropriate moments to strike. At the same time, the issue of Desdemona being responsible for her own condition might be a bit more complex. Certainly, she bears a level of responsibility, but she is victimized by both Othello's insecurity and Iago's machinations. Within both polarities, reasonable ends would conclude that there is challenge present.
When Desdemona first meets Othello in the play by William Shakespeare, she keeps a quiet profile to begin with, hovering around the men discreetly and just listening to the conversation of the great men around her, at table and so on. So she has time to assess the Moor and make observations about his character, personality and integrity. She hears tales about his bravery and his honour and finds him steady and entertaining. But what she maybe does not realise is that she is seeing him at his best, being lauded and praised for his actions of valour in battle. She has not yet seen him under the kinds of stresses which reveal the sorts of flaws and failings we all have. Othello's (low self-esteem and suggestibility) are devastating. She has not seen that he is easily manipulated, or guessed that this might come from Iago.