This act of kneeling is a visual confirmation that Othello has capitulated to the dark side and is now under Iago's Satanic power. The kneeling shows Othello's change of heart and vow to enact the vengeance Iago has so long desired, and his rising reveals his resolution to act on his violent thoughts.
Throughout the play, Iago has been working on Othello with slow drips to plant the suspicion that Desdemona is unfaithful to him, cannot truly love him, and is, more particularly, having an affair with Cassio, the man Iago most, after Othello, wants to injure.
Finally, in act 3, scene 3, Iago's evil plot comes to fruition. Through innuendos, protests that he couldn't possibly tell what he knows, and outright lies, Iago convinces Othello that Desdemona and Cassio have betrayed him, that Cassio must die, and that Iago must replace him as Othello's lieutenant.
Othello's kneeling and taking vows is both a sinister parody of a marriage vow and a parody of the oath of fealty a warrior takes to his...
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