How does Othello's character change throughout the play?

Unfortunately, Othello's character development over the course of the play traces a tragic fall from grace. 
Othello begins the play as a successful African general in the defense forces of Venice. Although he is separated from his colleagues by the color of his skin, he is still respected and considered a man of both courage and good professional judgment.
Unfortunately, Othello's outsider status leaves him vulnerable; his insecurity over his racial identity and how it is regarded by the Venetians creates within him a sense of social separation. While this seems to be mended by his marriage to the loving Desdemona, the fix is only temporary.
When Iago convinces Othello that Desdemona is cheating on him with his lieutenant Cassio, Othello experiences a transformation of character. Overcome by jealousy, he finds his mind consumed with doubt and misgivings about his previously stable relationship with Desdemona. His confidence falters and his insecurities begin to surface and unravel until Othello finally kills the very person he loves best: his wife.
Realizing that he has destroyed the most humanizing aspect of his life, Othello takes his own life. Thus, we see the tragic fall of a hero crumbling into a man defined by—and eventually destroyed by—his fears and judgments.