Iago is in many ways a metaphor for the Christian concept of the devil. He does not commit crimes himself. He tempts others, abuses their moral weaknesses, and persuades them indirectly to commit horrible crimes. While he claims to do this out of fear that Othello has slept with his wife, he presents very little evidence of this; it seems more likely that Iago enjoys hurting others for its own sake. This makes his character even more sinister, since he stands to gain very little from his horrible actions.
The name Iago itself means the planter. This is fitting, as he plants negative notions in the minds of others and allows them to take a tragic course. He does this by playing on Cassio's weakness for wine and on Othello's leanings towards jealousy. This method of indirect temptation for no reason other than a love of causing harm is the very essence of evil.