Iago is indeed a villainous character. He destroys a happily married couple. Through his own greed, he is offended when he does not receive the position of lieutenant. Hatred fills Iago's heart. He becomes evil in his actions.
Othello is set up by Iago's claims that Desdemona is indeed having an affair with Cassio. Although this insinuation is far fetched, Iago's plan works. He convinces the great Othello that his wife has been unfaithful.
To prove how evil Iago is, he uses Cassio and Roderigo as a part of his plan. Then in the end, Roderigo is killed, set up by Iago. Iago is so evil until he will use anyone in his path to get revenge for not being assigned the position of lieutenant.
Ultimately, Iago's plan of jealousy works. With Othello convinced of Desdemona's unfaithfulness, he murders Desdemona. With Desdemona's death and Emilia's confession about the handkerchief, Othello regrets his action and kills himself.
Due to Iago's evil nature, Othello is dead. Desdemona is dead. Emilia is dead. Roderigo is dead, and Cassio is injured. Sadly enough, all these innocent deaths are the result of Iago's evil nature to get what he wanted at any cost.
In the end, Iago would not speak, not even to pray:
Demand me nothing; what you know: you know.
From this time forth I never will speak word.
LODOVICO:What, not even to pray?
I consider Iago to be the most villainous character in literature, and critics certainly assert that he is the most vile of all of Shakespeare's creations.
Iago is a misanthropist - he hates all humanity: probably including himself. He uses everyone around him as puppets in his cruel game of destruction - a game which seems to be merely for sport, without real purpose.
Iago tells the audience that his destruction of Othello is a revenge for Othello having-
done my office.
That is, slept with Emilia, Iago's wife. There is little foundation for this. If Iago truly loved his wife aand was greatly offended by his master's actions towards his beloved, he surely would not have stabbed her so cruelly, or used her to procure Desdemona's handkerchief.
Iago wishes to destroy Desdemona too, and-
turn her virtue into pitch.
Again, this may be revenge for being cuckolded, but his engineering that Desdemona is strangled in the marriage-bed she was true to is a viciously ironic gesture.
Iago torments and destroys his 'friend' Roderigo - taking his money, his dignity, and his life. He kills Roderigo without remorse when he has served his purpose and poses a threat.
His torture of Othello is most compelling in its cruelty. Iago clearly enjoys his games with Othello, and revels in the torment of the man who deems him 'honest'. He even watches his master suffer the indignity of an epileptic fit, brought on by Iago's insinuations and poisonous words-
IAGO: Work on,
My medicine, work! Thus credulous fools are caught:
And many worthy and chaste dames even thus,
All guiltless, meet reproach.
Critics have long deliberated Iago's motives for his dark actions, with Samuel Taylor Coleridge categorising iago's actions as 'motiveless malignity'. Part of the fascination with Iago is that we cannot understand him, as his actions are neither explained nor reasoned.