Roderigo’s rage at Cassio is a part of Iago’s evil plan. He intends to disgrace Cassio in front of Othello. His incompetent behavior can lead to his dismissal from the lieutenant’s position. The lack of trust in Cassio can also make him suspicious regarding his relationship with Desdemona. So, Iago provokes a conflict between the two characters. Othello witnesses it and fires the lieutenant on the spot.
Detailed answer:
Iago hates the protagonist and is ready to do everything to destroy him. In the second act, he develops a plan that relies on convincing Othello of his wife’s betrayal. Insecure and vulnerable, the protagonist can be easily manipulated. Yet Iago doesn’t rely solely on rumors. His plan is more layered than that.
To make Desdemona’s adultery believable, Iago decides to ruin Cassio’s reputation. Besides, he is jealous of the young lieutenant. Othello promoted him to the position that Iago wanted for himself. In one scene, the antagonist manages to ruin Cassio’s career and dishonor him. Thus, Iago made his alleged affair with Desdemona plausible.
Being informed about Cassio’s problems with alcohol, the antagonist convinces him to drink:
If I can fasten but one cup upon him,
With that which he hath drunk to-night already,
He’ll be as full of quarrel and offence
As my young mistress’ dog.
(Othello, Act 2, Scene 3)
As a result, the lieutenant starts acting inadequately. He gets into a conflict with Roderigo, another Iago’s victim. The villain asked him to initiate this argument:
How now, Roderigo!
I pray you, after the lieutenant; go.
(Act 2, Scene 3)
Meanwhile, Iago leads Othello to the place of the fight. Witnessing Cassio’s behavior, he fires him from the position. Frustrated, Cassio starts trying to gain Othello’s trust back.
Attempting to recover, he asks Desdemona to talk positively about him in front of her husband. Iago does not miss this chance. He continues convincing Othello that his wife is cheating on him with Cassio. Desdemona’s flattering speeches about Cassio gives reasons for suspicion. By the end, Iago’s villainy wins. Othello believes in his gossip and murders Desdemona for being unfaithful.
Overall, provoking a conflict between Roderigo and Cassio is just a little part of Iago’s plan. He aims to destroy his opponent and is ready to do everything for that. Cunning Iago quickly detects everyone’s weaknesses. He has power over naive people.