Is Othello a victim of a destiny and/or forces beyond his control, responsible for his own demise, or was it a combination of the two?
I think the text of the play suggests that it is a mix of both. The Othello that we come to know through the action of the play is a supremely capable and powerful general very confident in his own abilities as a soldier and leader of men. He is too trusting in Iago but this is hardly a cause for concern on its own.
Yet when the Turkish fleet is destroyed and he no longer has a clear purpose in defending Cyprus from the dreaded Turks, he has nothing to occupy his time or his concern but Desdemona and the supposed cheating that Iago begins to tell him about.
The combination of his lack of confidence in navigating the intrigue and plots of court and people like Cassio and Desdemona and the fate that befalls him because of the sudden absence of an enemy combine to bring him to murder his beloved and then kill himself.