Explain Iago’s evil plan for Cassio and why he hopes that Cassio will also kill Roderigo. Why does his plan fail, partially?

Iago wants to set Cassio up. Iago is jealous of Cassio's promotions by Othello. He decides also to destroy the moor at the same time. He is going to orchestrate a rumor that Cassio and Desdemona are having an affair hence rendering Othello the dreaded "cuckold." Iago uses Roderigo's obsession with Desdemona to begin a fight with Cassio and get Cassio in trouble with Othello. Iago can't deliver on Roderigo winning Desdemona plus Roderigo knows too much.