Motivation?

"Some such squire he was That turned your wit the seamy side without, And made you to suspect me with the Moor." - Emilia (IV.ii.145-147)

Up to this point in the play, Iago had one of two motivations left to base his evil plot off of. Emilia knows this rumor went around, so she addresses it in front of Iago. She calls him out about believing the rumor about Desdemona and Cassio, but not the one about her and Othello. His motivation basically just vanished. So now is the proof that Iago is just a jerk. Motivation is described as the reason a character acts the way they do; I no longer know what Iago's motivation is. I think he might generally just be a heartless, abusive, power hungry person who enjoys tormenting people. (If you haven't figured it out yet, I hate Iago). Iago isn't used to Emilia actually speaking to him, let alone calling him out publicly, so I think that's why he gets so defensive at first mention. Iago's motivation, if he has one, has become to keep his plan going until everyone is dead or alone in life. What a nice guy. 

All of the people in this plan are so susceptible to rumors. They're so gullible, 

and it's causing them to go mad. Adele doesn't perfectly describe Othello, 

but I like this song so it's going on the blog.