The speaker is ill and keeps going insane over another guy's "vulture eye" so he kills him and then the police come and don't suspect a thing but there is a noise inside the speaker's ears so he tells them just to make it go away.
The tone is upset because the speaker is upset when he realizes what he's done just to get rid of that vulture eye, and now the never-ending heartbeat is getting louder by the second.
The word "trouble" stands out because it is bolded and contributes to the story. He was talking about the eye when he said "trouble."
"Yes! Yes, I killed him." I can picture the scene where the guy is talking with the police and then the noise gets too loud so he tells them and tries to make it stop
The author used a simile when he said the body was dead as a stone. A stone is not moving and grey, and it was the same with the body.
A possible theme for this story is that everything is not as it seems. The speaker thought it would be a good idea when he killed him, but it ended up haunting him instead.