Although he discovers it too late to save himself or anyone else, Roderigo is the first to see Iago as the villain that he really is.

Iago has done an effective job of convincing everyone that he is trustworthy. Up until act 5, scene 1, all his machinations have gone off perfectly. However, Roderigo's botched attempt to murder Cassio forces Iago to take direct action. Iago mortally stabs Roderigo, ostensibly in retaliation for the attack on Cassio. However, it was really done to prevent Roderigo from revealing Iago's involvement. Up until this point, Roderigo had trusted Iago. With his dying words, Roderigo seems to finally see Iago for the treacherous villain that he is, crying "O damned Iago! O inhuman dog!"

Roderigo never comes to understand the extent of Iago's villainy. It is Emilia, Iago's wife, who first truly realizes how wicked Iago is. In act 5, scene 2, Othello has just killed Desdemona. He explains to Emilia that he did this because he believed Iago's insinuations that Desdemona and Cassio were having an affair. Emilia knows this to be false and quickly realizes that Iago has been spreading these deadly rumors. She tells Othello,

... may his pernicious soul
Rot half a grain a day! He lies to th heart.

When Iago enters the scene, Emilia directly accuses her husband, who responds by killing her.