In the play Othello by William Shakespeare, is Iago justified in his actions or does he hurt Othello, Cassio, and Desdemona just because he could to satisfy his ego and prove his place in the world?

It is difficult to posit any evidence that justifies Iago's actions in Othello. At the beginning of the play, he explains to Roderigo why he has decided to torture and deceive Othello: he feels that he has been passed over by promotion in favor of Cassio, who "never set a squadron in the field" and that this is unfair. While he may be right, this does not really seem sufficient justification for his attempts to systematically destroy Othello's mental stability.
We can certainly argue that Othello, not Iago, is responsible for the ultimate death of Desdemona—it is Othello, not Iago, who decides to "put out the light" and murder her. However, it is inarguable that it is Iago's actions which stretched Othello's sanity to breaking point and caused him to want to kill Desdemona, convinced that she had betrayed him. Iago seeks to hurt everyone around him, even his own wife, in this play, and the justification he offers seems certainly insufficient.