Lyric discussion by HateSolstice 

Cover art for Nightmare lyrics by Avenged Sevenfold

I really don't believe this song is about anyone in particular. It seems quite clear to me that the subject of the song is simply being taught the lesson of consequences for committed actions. Perhaps the subject believed what they were doing was the right thing, lied to with the truth twisted by whatever "demons" haunted them. Or perhaps the subject is literally mentally ill, and is unaware of whatever actions they committed.

However, the line I find most curious is thus:

"So sedated as they Medicate your brain And while you slowly Go insane they tell ya "Given with the best intentions Help you with your complications""

Another possibility is that the subject is undergoing some sort of treatment for mental health, and has either been told by their caretakers that what they have done is wrong(and due to the medicated state, believes them). Or perhaps these are the same voices that they have succumbed to in previous verses due to the medication.

There's many ways to take this song, all of which are rather interesting. It almost feels to me like a "Welcome Home(Sanitarium)" message, but with perhaps a much darker protagonist/subject.