In Act II, Cassio cries, “I have lost the immortal part of myself, and what remains is bestial.” How does this sentence apply to Othello?
Because Othello similarly loses the immortal part of himself, leaving something far more uncivilised, savage, animalistic and bestial.
What do we mean by the immortal part of himself? Well, it could mean lots of things. The part of you which doesn't die. Is that the soul? The religious awareness - the bit that goes to heaven? Or is it just simply a person's goodness?
In any of these cases, it's obvious that, as Othello submits to Iago's suggestions, and believes that his good, noble, angelic wife has cuckolded him, the veneer of his Christianity vanishes into a bleaker more fundamentally bestial worldview. "Why did I marry?" he asks, and - by the end of the play, he is calling for "blood", and raving about "goats and monkeys".
Although Othello is strong and confident at the beginning of the play, he allows himself to be manipulated by Iago into a jealous and angry man. Eventually, his character disintegrates into a murderous lover and finally into a suicidal man who can only say that he was ""one that lov'd not wisely but too well". Yet he has taken the life of his innocent wife and then, when confronted with the truth, take his own life. According to Elizabethan values he has lost the "immortal" part of him. He is damned and all that is left is the bestial or unredeemed body of a killer.