Almost every time I have a question about the plot of a Shakespeare play, I find that William Shakespeare had it covered.
For example, when Othello confronts Desdemona with her supposed adultery she asks him to send for Cassio to “Let him confess a truth”. Othello tells her that Cassio‘s “mouth is stopped” and that “Honest Iago hast ta’en order for’t”. But did Othello have any reason to think Cassio was dead beyond the fact that he had ordered Iago to kill him?
I could not think of any other “evidence” that Othello might draw upon, but I had forgotten the little incident in Act 5, Scene 1, just after Iago has struck Cassio in the leg from behind:
CASSIO: I am maimed for ever. Help, ho! murder! murder! Falls
OTHELLO: The voice of Cassio: Iago keeps his word.
RODERIGO: O, villain that I am!
OTHELLO: It is even so.
CASSIO: O, help, ho! light! a surgeon!
OTHELLO: ‘Tis he. O brave Iago, honest and just,
That hath such noble sense of thy friend’s wrong!
Thou teachest me: minion, your dear lies dead,
And your unblest fates hies. Strumpet, I come!
Forth of my heart, those charms, thine eyes, are blotted;
Thy bed lust-stained shall with lust’s blood be spotted.
This is just one of many little examples of the care Shakespeare takes with his plots. How would Othello “know” that Cassio is dead? By overhearing the fight and the cries of Cassio, apparently in his death throes. Hence this ironic little scene, which also serves to strengthen Othello‘s resolve to kill Desdemona.