OK, this is a very fascinating topic to discuss. Firstly, I have to say that I don't agree that Iago is not evil, but it can always be interesting to try and turn such issues on their head and look at them from a completely different perspective for one moment. To answer this question, I would suggest that you would have to start by looking at the cause of Iago's hatred for Othello. If we examine Act I scene 1 and how Iago justifies his actions and his hatred of Othello, we can see that Iago does have cause to feel seriously disgruntled:
Three great ones of the city,
In personal suit to make me his lieutenant,
Off-capped to him, and by the faith of man
I know my price, I am worth no worse a place.
But he, as loving his own pride and purposes,
Evades them, with a bombast circumstance
Horribly stuffed with epithets of war,
And in conclusion
Nonsuits my mediators.
Let us try to put ourselves in the shoes of Iago for one moment. If we believe his words, Iago has been turned down for a promotion that he knows he is capable of carrying out and that other "great ones" believe he is capable of carrying out as well. How would you feel if this happened to you? If we believe that Iago is a man who, as he says, "knows his price" and knows what he is capable of doing, Iago must have felt horribly frustrated and angry at the way in which Othello picked Cassio over him and ignored the opinion of the three senior members who championed him. Is it possible to argue that Iago, rather than being simply evil, becomes overpowered by his anger which leads him to commit acts that he never would have wanted to commit? Trying to look at this speech from Iago's point of view does help us to feel sympathy for him, as anybody who has faced a similar situation can no doubt identify with him.