why does she take no steps to protect herself in act four scene three

Desdemona is too good and too devout to do anything about it. The "Willow Song" and her tale of her mother's maid also foreshadow Desdemona's death; yet her resignation is still strange. She is not trying to fight it; she seems like a totally different woman than the one who stood up to her father and the Venetian nobles in defense of Othello. Desdemona, although she is good, is suddenly depicted as being meek; this sudden shift in her character is strange, and the source is unknown.

Desdemona is almost too good to live; indeed, had she admitted some fault or sin to Othello, it would have shattered his view that she was merely pretending to be good, in order to hide her treachery. Her character is parallel to that of Hamlet's Ophelia; both are good, virtuous, obedient, but both are subjected to tragic fates in spite of - or because of - their innocence. Desdemona's fate is unfair and unearned, yet she is the martyr of the play, the tragic female heroine who ends up being sacrificed to satisfy the fates.