The Question and Answer section for Othello is a great resource to ask questions, find answers, and discuss the novel.
This depends on where in the play you are referring. Generally Iago is pretending to be Desdemona's friend who is just as confounded by Othello's strange behaviour. In reality Iago is orchestrating the chaos that he has unleashed upon the moor.
This is such a broad question for this short answer space. Discussing human nature in Othello depends on a variety of characters and character traits. I can comment on specific characters if you like.
Emilia's repetition of words speaks to her shock and confusion. She cannot believe what Othello tells her about her husband, and yet, here Desdemona lays dead, and Othello claims that her husband reported Desdemona's behavior.... all lies.