Spoiler question
How was the captain's death explained to the crew? The killing may have been justified self-defense, but the killer would not have been able to explain that, being mute and illiterate. It is implied that the already suspicious officers would have figured out the captain was a maniac, but it is not made clear how.
Also, if the mute could not read, how would he know the significance of the telegram, and that it should be passed on the someone? I don't remember him listening in on any conversation that might have let him know that.