What was Big Willy Shakes going for?
We know what you're thinking: where is the evil, scheming part? Isn't Iago supposed to be crueler? Ah, Shmoopers, you're onto something there. The genius of Iago's plan is that no one suspects it. Everyone goes around town praising how honest and trustworthy he is, when in fact, he's the most deceitful one of the bunch. (That's what we call irony.)
So when he says "O beware, my lord, of jealousy; It is the green-eyed monster which doth mock the meet it feeds on," what he's actually saying is "I hope you become jealous and kill your wife, because that would, ironically, fulfill all my plans. Ha!" Okay, so we added the "Ha!".
In fact, a lot of the power behind this quote lies in the fact that Iago is being incredibly ironic here. We know he's malicious and nasty. We've heard of his plan to take Othello down. And we even know how he's going to do it. But Othello doesn't know any of that. In fact, he thinks Iago is just about the best guy he's ever met.
We in the audience know not to trust him. We see how Iago uses dishonesty to convince Othello that his wife is unfaithful, all while pretending to be looking out for the best interests of his so-called friend.
You might think this quote has some truth to it. And you'd be right. Jealousy does make people turn into monsters. But that's not quite the whole point of it. In reality, Shakespeare is warning us not to trust anything Iago says. Ever.
A few years earlier in The Merchant of Venice, Portia says "green-eyed jealousy" (3.2.110). Looks like Shakespeare couldn't get the idea of green and jealousy out of his head. Now that he coined this phrase in Othello, we can't get it out of our heads either.