One could argue that both destiny and individual choices result in Othello's eventual downfall. Othello's race plays an important role in his relationship with Desdemona and is something controlled by destiny. Othello cannot change the fact that he has dark skin, which makes him an enemy in Brabantio's eyes. One could assume that if Othello were a white man, Brabantio would not have an issue with Othello marrying his daughter. Othello would also not feel like an outcast in Venetian society, and his race would not affect his confidence. Fate also influences Othello's visit to Cyprus. If there had not been a violent storm that destroyed the Turkish fleet, Othello would probably have had less time to focus on Michael Cassio and contemplate his wife's infidelity. Destiny also plays a role in Desdemona accidentally losing her handkerchief, which is very important to Othello and presumably confirms his suspicions.
Despite the role of destiny and fate in Othello's life, he makes several independent decisions that lead to his downfall. Othello makes the mistake of completely trusting Iago and allows his jealousy to obscure his perspective. He also allows his low self-esteem to affect his thoughts regarding Desdemona and does not use proper judgment when analyzing the entire situation. Finally, Othello makes the independent decision to murder his wife and ends up killing the love of his life at the end of the play.