How and why does William Shakespeare create sympathy for Othello?
Othello is human. As we all are from time to time, he is is led astray by false friendships, misplaced trust, and logical fallacies.
The first human and fatal error Othello makes is trusting the evil Iago. Othello calls him "honest Iago," trusting his confidant with the news of his elopement, but Iago plots immediately to betray his friend and tell Othello's father, the Duke.
Othello's judgment fails him again when he disregards his own reasoning, and is swayed by his father's opinion of Desdemona. The Duke plants the idea into Othello's heart that she may be untrustworthy. "Look to her, Moor, if thou hast eyes to see;/She has decieved her father, and may thee" (1.3.291-92.) True, thinks Othello, she *did* decieve her own father...why not me??
This poison seed, wrong from the start, breeds weeds of distrust. How society feels about Othello is the same way any of us may react to such a horrific misplacement of blame. We feel outrage and sorrow, but perhaps also are glad that our own mis-steps did not become so disasterous.