The character of Iago in this play is a bit of a mystery in that we never are truly told what his reasons for wanting to ruin Othello are. What we do know, however, is that he is consumed with his jealousy and hell-bent on achieving his revenge.
One possible reason for his jealousy might be that he was passed over for promotion in favour of Cassio. Iago confesses to Roderigo that this is the reason for his hatred of Othello:
One Michael Cassio, a Florentine,
(A fellow almost clamn'd in a fair wife)
That never set a squadron in the field
But he, sir, had the election...
Therefore one of his motivations for his acts are being blatantly passed over for promotion by Othello, even though he is more experienced than Cassio, who has never seen active service.
Another, more compelling reason for his jealousy is his suspicion that his wife Emila has had an affair with Othello:
...I hate the moor;
And it is thought abroad that twixt my sheets
'Has done my office. I know not if it be true;
Yet I, for mere suspicion in that kind;
Will do as if for surety.
Despite having no tangible proof of this supposed entanglement between his wife and Othello, the very fact that he suspects that it could have occurred is enough to continue Iago on his course to destruction.
Alongside this reason, we can infer that Iago is extremely jealous of Othello's ability to woo the young and beautiful Desdemona. Perhaps Iago secretly lusts after Desdemona, and he is unable to bear the thought of that "black old ram" as he calls Othello, obtaining what he himself desires:
It cannot be that Desdemona should long
continue her love to the Moor... She must change for youth.
When she is sated with his body, she will find the error of her choice.
Above all of these reasons, however, it is clear that Iago will take any opportunity to commit acts of evil. Even if these reasons did not exist, he would still set out to destroy Othello for this reason. He sees his machinations as a kind of game: "Let us / Be conjunctive against him. If thou can'st cuckold him, thou dost thyself a pleasure, me a sport."
Iago therefore can be said to be a psychopath, he is not able to feel guilt or remorse and has a lust for evil that goes beyond his control.