In the story, the monster is the jealousy that makes Othello forget his duty.
Step-by-step explanation:
Othello is rather sympathetic character but he is marred by jealousy of a person who he does not trust and this jealousy is fulled in him by someone with an ulterior motive
He is smart enough to follow his duty and understand that he is being played but he gets blinded by the trust he places on his compatriot.
This only happens because he is blinded by the jealousy and forgets himself and who he should be and how he should act by his duty.
He trusts wrong influences and brings ruin to his life.