Hate and love can only be deemed justified by the person with thte emotion. Does Iago feel as though he has just cause to hate Othello in his mind? He is able to bring up many reasons for hating Othello, therefore Iago feels as though he is just in his emotion.
Iago hates Othello for some of reasons. First reason could be that Othello promoted Cassio in his place; however, Iago wants it and he cosidered himself more superior than Cassio for that possition but when Othello promoted Cassio instead of Iago neglecting Iago's calibre. Hate for othello increases in Iago's heart. Secondly, Iago's thinks that othello has illegal relationship with her wife. Also, Iago hates him because Othello married Desdemona. Othello got the wife and according Iago Othello is not capable for such a beautiful and caring wife like.., Desdemona. He also hates him due to the differences in their race. Iago hates othello but the revenge he took is quite inappropriate. In his hate, he ruined the life of a married couple.
There are some who feel that Iago lost his promotion to Othello. Then Iago looses out to Cassio and doesn't get the promotion he feels he deserves. Also, to put it bluntly, Othello got the girl. Iago is obviously a very intelligent man, but decides to put his energies into bringing down Othello - that smacks of nastiness.