What specific line in Act V supports the theory that Iago is a devil incarnate? Who says the line?
In Act 5, scene 2 of Shakespeare's Othello, the title character realizes, when it is too late, that he has unjustly killed his faithful wife, Desdemona. When Iago, the driving force behind Othello's downfall, is brought in, Othello says,
I look down towards his feet; but that's a fable./If that thou be'st a devil, I cannot kill thee (5.2.330).
Othello's mention of looking at Iago's feet suggests that he's looking for cloven, or hooves, as many believe that the devil has cloven feet. Further, he asks the officials in Cyprus to ask Iago, whom he refers to as "that demi-devil," why he has ruined Othello's life.
At this point in the play, Othello has obviously realized the extent of Iago's evil. Thus, he has no other explanation for Iago's behavior than to label him as a devil.
In Act V of Othello, after Othello kills Desdemona and Emilia reveals Iago's villainy, Iago stabs Emilia and tries to escape. Of course, it is too late, and he is brought back into the room. At this point, Othello says to Cassio:
" Will you, I pray, demand that demi-devil/Why he hath thus ensnared my soul and body?"
He does not address Iago directly; at this point he cannot even look Iago in the face. Then Iago responds that from that moment, he will no longer say anything, and he does not say another word. This question of Othello's could be said to support the argument that Iago is a devil incarnate.