Short answers, pulled from the story.
Iago stands as a fictional character who serves Othello for years as standard-bearer before vowing revenge after being passed over for promotion to lieutenant. He engineers the downfall of both Michael Cassio and Desdemona through deception and manipulation.
Giuseppe Verdi introduced an aria called Credo in un dio crudel in Act II that has no counterpart in Shakespeare's original text. This piece reveals Iago believes he was created by a cruel god and that his evil fulfills destiny rather than arising from random malice.
Micheál Mac Liammóir starred in Orson Welles' 1951 film version while Andy Serkis and Daniel Craig took on the role in modern adaptations. Other notable performers include Frank Finlay in the 1965 film version, Bob Hoskins in season four of BBC Television Shakespeare in 1981, and Josh Hartnett in the 2001 TV movie O.
Samuel Taylor Coleridge described the character as suffering from motiveless malignity suggesting he wreaks havoc without ulterior purpose. Fred West contends that Iago is an accurate portrait of a psychopath who is devoid of conscience with no remorse while A.C. Bradley writes that such evil appears compatible with exceptional powers of will and intellect.
Iago steals a handkerchief from Emilia to use as central evidence claiming he saw it in Cassio's possession. This triggers jealous rage in Othello leading him to kill Desdemona who remains innocent of all charges.