The 'green-eyed monster' — Iago plants jealousy in Othello and watches it destroy him. Shakespeare shows jealousy as one of the most destructive human emotions: once seeded, it distorts everything.
Othello's race is central to the play. Iago and Brabantio use racist language about him. Shakespeare gives Othello extraordinary dignity and eloquence. The play explores what it means to be an outsider in a society that fears difference.
Iago is trusted by everyone and is entirely untrustworthy. Othello calls him 'honest Iago' repeatedly. Shakespeare shows how destructive misplaced trust can be.
Characters in Othello are defined by their reputations — and Iago understands how to destroy reputations without leaving evidence.