The film is a reflection as to how such a great tragedy could have occurred. We see this clearly in Welles' shot of Venice reflected in a canal, after we already have seen that Othello and Desdemona are dead. Thus, this film is a cautionary tale to anyone in power who receives counsel from another, and for anyone who chooses to believe the words of another over those of their spouse. It is a tale of caution for anyone who seeks to attain justice on their own. And it is a tale of how easily we allow ourselves to be manipulated and make decisions based on emotion.
Shakespeare's tragedy has withstood the test of time because it deals with themes that are to this day relevant, including race, betrayal and love. Othello must overcome the reality of race being the reason Brabantio cannot believe his daughter has married him. This type of treatment for one's entire life is a great burden to withstand, and it contributes to his willingness to believe Iago's lies as he see Desdemona with a man of her own skin color, a man whom her father would likely call no counsel in order for her to marry. These matters are not blatantly spoken but we see them in the film as they become the undertones for why Othello could believe Iago.
Finally, taking justice into our own hands is a great danger that we face each day when presented with hardship. It is not for one to determine the outcome, whether one is to live or die, but it is a matter to be judged rightly with all evidence taken into account. This is not done, and Othello rushes to "put out the light." He is so blinded by his rage that he doesn't dare believe anything that Desdemona is saying to him though it is the truth.
