Your thesis will relyupon which interpretation you take of Iago's situation. If you want to make an emotional argument to defend him, you will need to cite examples of his being passed over for a position that he had supposedly worked years for and that his "friend" Othello didn't even seem to notice his hurt feelings. You could also point out that his wife does not seem to respect him or truly love him; so his home issues could affect his choices. Though there is no proof of this in the play, you could also blame Iago's actions on a bad childhood--something our justice system often sees; for how could someone be so evil and vengeful without provocation? Your thesis statement could be similar to: Shakespeare's Iago, a seemingly malevolent villain, is justified in his actions because of a difficult home life and his employer's unfair treatment of him.
A better argument is a more logical defense. You could argue that Iago suffers from narcissism--a "disease" that causes one to focus solely on himself. In all of his speeches and actions, Iago expresses no concern for anyone else, not even his wife. You will be able to find numerous examples of this in the play.