The downfall of Othello can be attributed to many factors and some of these are inter-related. The villainy of Iago is very effective in making Othello believe the lies and deceptions. Othello has many weaknesses in his character which make him very prone to jealousy and becomes very obsessive. Othello seems to have little real understanding of love and its meaning, which contributes to his downfall. He doesn't feel as though he completely fits in to the Venetian society and this leads to feelings of insecurity where he sees himself as an outsider. He also finds himself a long way from Venice and is free from the laws and ways of that civilised society, which has an influence on his downfall. .
Iago, the strong and persuasive villain, is in many ways responsible for the downfall of Othello. While Iago's motive are not clearly stated, he does express his hate for Othello, "I hate the Moor" (Iago: act ----) and implies he wants revenge on Cassio and Othello because he didn't get the position of left lieutenant. Iago uses his ability to understand different people, identifying their motives and weaknesses, and using this skill to manipulate them for his own gains. His deceptive nature is evident through out the play with constant referrals to him as "honest Iago". It is also shown in his relationships with nearly all the major characters of the play including Roderigo, Cassio, Emilia, and of course Othello. .
Iago identifies the weaknesses in Roderigo, that is his undying love for Desdemona, and his kind-hearted gullible nature. He manipulates Roderigo by making shallow promises, such as how Desdemona will fall in love with him, and also by appearing to be a kind, trust-worthy friend. The irony in this is that Iago intentionally kills Roderigo. .
Iago also identifies Cassio's flaw as his importance of his reputation to him. He then plays on this weakness by making him appear an irresponsible alcoholic, "'Tis evermore his prologue to sleep.