What does Iago's plan to set Iago and Cassio against each other in deadly combat tell us about Iago

First and foremost, he is deceitful and manipulative.

Iago again proves himself a consummate actor. This scene again brings up the theme of appearance vs. reality; for though Iago claims to know nothing of this battle, and be merely discovering it, he is the mastermind of the entire situation. Iago is many selves in this act; he is friend and advisor to Roderigo, then betrayer and murderer of Roderigo, consoler of Cassio, and the lead officer in this crisis. He uses misrepresentation to fill each of these roles as best he can, and not let the others know of his true plans and character.