Why do Iago and Roderigo decide to go and disturb Brabantio? What can they hope to achieve by annoying him?

Roderigo just wants to prevent Desdemona from marrying Othello. Iago has more sinister plans. He wants to create conflict between Othello and white nobility. He wants to instill a sense of paranoia in Othello that Iago will nurture and cultivate throughout the play. Both Rederigo and Othello know that Brabantio will not be pleased that his daughter is marrying the moor.