Iago is likely the best subject for a discussion of manipulation and duplicity in the play given that he is a vortex of both.  Iago's ability to manipulate everyone around him, whether it is his general, Othello, or his foolish boy Roderigo, is absolutely masterful.  From the outset, he gets each character to do as he pleases, including Othello, Roderigo, his wife, Desdemona, Cassio, and the list goes on.  And he does so almost always under the guise of loyalty to Othello, Cassio, Desdemona, and even his wife, though she gets short shrift.

Yet he also makes it quite clear to the audience that all of that loyalty is the height of hypocrisy, that he serves a man merely to do his turn upon him.