Iago’s soliloquy at the end of the first act can be paraphrased as follows:
“I am planning to use this fool, Roderigo, to achieve my own goals, because I cannot waste my time with the idiot. I have hatred for the moor, especially with the rumor doing the rounds that he slept with my wife. The rumor may not be true, but I will act as if it were true. I know Othello respects me, which makes it easier to execute my plot. I will take the ranks of Cassio and feed my ego at the same time.”
This speech expresses bitterness and hatred, and it seems that Iago wants to get revenge on Othello, whom he believes has been sleeping with his wife. This means that Iago is up to no good and his intentions are evil. This proves his general character in the play, where he thrives on the flaws of the other characters.