Not Cassio kill’d! then murder’s out of tune,
And sweet revenge grows harsh.
– William Shakespeare
Othello, Act 5, Scene 2. Othello almost gives himself away when he hears that Roderigo has been killed by Cassio, who remains alive. Othello realizes that his plan for revenge on Cassio has fallen apart and the wrong man has been killed. Iago had promised to kill Cassio for Othello. Describing the murder as being "out of tune" is a metaphor for it not being right or lawful.