Iago’s third soliloquy (lines 310-36) has two main parts. In the first part (up to line 324), Iago talks about his use of truth to declare himself innocent of being a villain. He proclaims that he merely states what is obvious and true. He is saying that he is innocent of any wrongdoing. He adds that his advice to Cassio (to ask Desdemona to speak to Othello on his behalf) is both logical and sensible. And it is. Desdemona does have control over Othello and is able to convince Othello of just about anything (including hiring Cassio back as his lieutenant).
The second part of the soliloquy, though, moves in the completely opposite direction. Here, Iago declares himself as a most deceitful villain. The second part begins with the expression “Divinity of hell” (line 324). The word divinity refers to theology or religious philosophy. Iago is declaring that his philosophy is the philosophy of the devil. His philosophy is truly evil. Iago explains that the best villain or devil is one who uses fraud, deception, and hypocrisy:
When devils will the blackest sins put on, They do suggest at first with heavenly shows.
(325-26)
One may perhaps think of a successful conman who pretends to be honest and caring and helpful, but who then turns out to be a crook who robs someone of all of his money. One might also think of the modern evangelist or minister (or even Chaucer’s Pardoner) who preaches to people that they should give up their greed and give all of their money to God. Of course, the money actually goes into that minister’s pocket for his own personal use. Iago is like the deceptive conman or the fraudulent minister. He uses good and honest words for an evil and dishonest purpose.
Cassio, of course, does not know that Iago will be hinting to Othello that Cassio is having an affair with Desdemona. So, the more Desdemona asks Othello to help Cassio, the more suspicious and angry Othello will become. And, all during this time, Iago will pretend to be a friend and supporter to both Cassio and Othello.