For many audiences, Othello is the most touching and intimate of Shakespeare's tragedies. The play focuses on three characters, two of whom are happily married to each other, and a third who wishes to destroy that marriage. Such destruction is not carried out as part of some grand design, such as to win territory or power or wealth. It is carried out for its own sake, and the seeming pointlessness of this vengeance contributes to the tragedy's poignancy.
The source of Othello is Giraldi Cinthio "Tale of the Moor", from a story collection known as the Hecatommithi ( 1565). Cinthio's story is a straight melodrama, and its essential features are retained in Shakespeare's play. The Moor is tormented over suspicions of his wife's infidelity, and his ensign causes these suspicions to manifest themselves in violence. Quite a few details are changed, however, and their alteration partially indicates how Shakespeare was able to take a seamy story and elevate it into poetic tragedy.
In Cinthio's tale, the unnamed Moor and his jealous ensign ( Shakespeare's Iago) together kill Disdemona [sic], then try to make the murder appear to be an accident. The Moor is captured, tortured, and exiled, and eventually slain by Disdemona's family. The ensign dies during torture for another crime. Shakespeare's plot, on the other hand, concentrates on the provocation to murder, and the time period is compressed into a few days. Furthermore, the qualities of jealousy and suspicion are dramatized with singular intensity.
More important are thematic changes. Cinthio's ensign has a clear reason for his destructive actions: he is jealous of the Moor, and seeks Disdemona for his own. In Othello, Iago's motivation is a mystery. Another crucial adjustment is the elevation of Othello to great stature and dignity so that his manipulation by Iago has more powerful consequences that pervade the surrounding society. But although the play does have political implications, these are subordinate to the love conflict.
The problem of Iago's motivation begins right at the start of the play. Coleridge attributed Iago's behavior to "motiveless malignity," but such analysis is ultimately unsatisfactory. People act as they do for reasons, and so do well-conceived characters. Surely the play provides evidence of Iago's reasons.