Why do Cassio and Roderigo fight?

Cassio and Roderigo’s fighting in Othello is part of Iago’s plot, and it helps destroy Cassio’s reputation before later turning deadly for Roderigo.

The first fight happens during the celebration in Cyprus. Iago gets Cassio drunk, then sends Roderigo to provoke him. Cassio chases Roderigo onto the stage, and when Montano tries to stop the fight, Cassio takes offense and fights Montano instead. Cassio wounds Montano. Othello arrives and demands to know what happened, and Iago gives a misleading account that makes Cassio look guilty while pretending to protect him. As a result, Othello dismisses Cassio from his position as lieutenant. This is exactly what Iago wanted, because Cassio’s disgrace lets Iago push him toward asking Desdemona for help, which Iago then uses to make Othello jealous.

Later, Roderigo attacks Cassio outside Bianca’s house because Iago has convinced him that killing Cassio will help him win Desdemona. Cassio’s armor protects him from Roderigo’s thrust, and Cassio wounds Roderigo. Then Iago sneaks in from behind and stabs Cassio in the leg before running away. When others arrive, Iago pretends to discover the crime, identifies Roderigo as one of the attackers, and kills him to keep him from exposing the plot. Cassio survives, while Roderigo dies.

Both fights show Iago channeling other men’s jealousy, shame, and ambition into violence. Cassio loses his office in the first fight, and Roderigo loses his life in the second, while Iago tries to stay hidden behind the chaos he created.