sad metal...

A couple of song recommendations that are about death, that I really like:

My Dying Bride - For My Fallen Angel
Anathema - One Last Goodbye

Both very sad. Read the lyrics as well.

However, if you want some of the most sad and emotional music, but not really about death, then check out Empyrium. I'd second the Agalloch recommendation too.
 
A very sad song actually is "Run of the Mill" off Judas Priest's 1974 debut Rocka Rolla. It describes an old man, about to die, wondering if his life was worth it and if he accomplished anything at all. One of the saddest songs I have ever heard.

Also "Solitude" by Candlemass: "Please let me die in Solitude"...that line gets me every time.

And who can forget the classic "Hallowed Be They Name" by the might Maiden. Although it's fairly upbeat musically it's a powerful yet sad song.
 
Life of Agony - River Runs Red

Some people may call them hardcore, but this album they really weren't.