Before you dwell into this question, it is important to know what a Psychopath actually is. What many people think of when they hear the word "psychopath", most will think they go on murdering sprees because that's what psychopaths do hence they are "Psychos" yadadada...
No. This is not what it means, and a psychopath rarely ever kills anyone. A Psychopath is when someone lacks any emotional response in the emotions of other people. Simply put, Psychopaths think noone else matters but themselves. They think they are the king of the world and everyone should bow down and kiss thy feet.
Iago is a psychopath because he does not think one bit about other people, except about how he can use them to benefit only himself. He uses his wide knowledge on human psychology and manipulation to get what he wants. He is "honest and kind" to those who he knows he can benefit himself from, while completely monstrous to those he has no concern or interest in, for example, Emilia, his wife, who Iago completely treats like dirt. He seems like the "Honest and kind Iago" to the following important people:
Othello: because he wants Othello to destroy himself through his maddening jealousy of Othello's wife Desdemona
Cassio: because he wants Cassio's place as second in command, and wants Cassio to be a scapegoat for Othello to suspect Desdemona's infedility with Cassio (which never happened)
Roderigo: because he can use this dimwit to make him rich and fight Cassio
So in the end, Iago's plan was giving "honest advice" while manipulating these people so Roderigo will hate Cassio and attempt to kill him, Othello will hate Cassio so that Othello would kill Cassio (that was the plan anyway, it didn't happen), and Othello would destroy himself so in the end Iago would be General. He.. is.. a.. psychopath.
If you want a real life example of a real psychopath's opinion of everyone https://www.youtube.com/watch?v=aKPzEToVzXI