Othello is too trusting of the wrong people. He believes Iago when Iago plants seeds of Desdemona being unfaithful. Othello becomes outraged with jealously. This becomes his downfall.
Othello suffocates his fair and innocent bride Desdemona. After the truth is revealed about Iago, Othello kills himself.
Othello's fault was in trusting the wrong people. He trusted Iago over his wife.
This ultimately leads to Othello's downfall. He should have questioned Iago's motives. He should have trusted his beautiful and innocent wife.
Perhaps, he could have spared the deaths of two beautiful and innocent people--his own life and Desdemona's.
Othello has two main weaknesses- he is too trusting and he also is consumed by jealousy.
He trusts Iago and falls prey to Iago's lies. He does not question Iago's information in a rational manner, thus falling prey to his other weakness- jealousy. Iago warns him that jealousy is a "green-eyed monster" but he continues to become jealous and this leads to the destruction of his marriage, as well the death of Desdemona and Othello himself.
When jealousy takes over Othello's character, chaos takes over his life. He told Iago that when he fell out of love with Desdemona "chaos" would return. Because he believes Desdemona has been unfaithful, the chaos takes over his life and marriage.