What has changed since Act I Sc. 3, in Iago's explanation of why he wants revenge on Othello and others? (II, 1, lines308-334) Why has it changed?

I don't think Iago ever really knows exactly why he wants revenge on Othello: he is just evil. Initially he claims that Othello passed him over for advancement but it becomes pretty clear he just likes chaos and to see Othello suffer.