Part of what makes this so challenging is that little, if anything, is clear from Shakespeare's drama.  There is a struggle to identify concrete and distinct realities that come out of the play, but some do.  The first that does is the idea that Iago has to bear a great deal of responsibility for all that happened.  Iago's machinations and his calculations are responsible for nearly everyone's suffering.  With this being said, I think that Othello has to bear some guilt for allowing things to have spiraled to such a condition.  Othello would have to be seen as guilty for not being able to open dialogues with those he suspected.  Instead of capitulating to fear and doubt, Othello could have simply spoken and confronted Desdemona or Cassio and not succumbed to Iago's insinuations.  I am not sure Desdemona is worthy of being considered guilty because she suffers such a great deal.  If there is any blame for her to bear, it might have been in the same condition as Othello in that there is so little open and honest dialogue between them and so much more hidden in the shadows.  Yet, I think that her inability to be transparent with Othello stems from his unwillingness or incapability to do this with her.