O how the mighty have fallen.
Well to me Othello’s further progression into madness is mainly due to Iago’s meddling with Othello’s life. Othello is this very gullible, trusting person who is willing to believe anything people tell him. Now that being said i do think that it was a little bit of Othello’s own jealousy that makes his green eyed monster grow even more. He took things way to far later in the play but at the start i do see this as Iago’s own doing. In my mind I think that if Othello found out about Cassio having the handkerchief Othello would ask Cassio or Desdemona for their side of the story first and listen to reason but because Iago is there sitting on his shoulder whispering things into his ear he starts to go mad. Normally he would trust his wife and his friend he would not allow anger to best him so easily. Who could believe it but again Iago starts to meddle by telling Othello to listen in a nearby bush then summons Cassio and starts to make it sound like he and Desdemona are having an affair. One of the main things that I think makes Othello so angry is that Cassio makes it sounds like Desdemona is just an annoying wench that follows him around. Again the normal Othello would have questioned this not wanting to jump to conclusions but because Iago has been pushing and twisting Othello’s mind he is willing to do anything to get revenge. Soon Othello’s hatred starts to pass the hatred of Iago. Iago really is the main reason why Othello has turned into this monster bent on revenge and killing because at the start we can see Othello is this well liked man that trusts his wife and everyone likes him. So i mainly see this as Iago’s fault but there is also some blame that can be place on Othello’s shoulders. I think at any point before hearing the conversation between Cassio and Iago, Othello could have stopped all this just by simply talking to someone else other than “honest Iago”.
So yeah all in all i say 80% Iago 20% Othello