The raven represents evil in a person and how it can affect you.
The narrator loses a loved one and then a crow shows up and won't leave him alone. The raven never takes the pain away and it always leaves him lonely. This is foreshadowing that he was depressed and the raven is symbolizing the evil in him and the only way to get rid of that evil is to “cut” it off.
I can infer that the narrator killed himself so that the burden and evil in him couldn't control him anymore.
The tone from beginning to end changes, at the beginning everything seems calm and collected. Then the story changes into the death of someone and a raven making someone show melancholy and the only way to get rid of this is to end their life which is another tone of depression.
Like i said in the last section the tone changes throughout the stroy from Calm, to Melancholy, to Depressed, and then to Death.
Evil can take over the body so don't invite it in.