Plenty of films have had dark endings, but none as unexpected as this, in a BOND film for crying out loud. It happens, the film enRAB, goodbye. Totally unthinkable for any blockbuster made since the 70s, particularly a Bond...
I was watching this t.v movie a month or two back about America in the immediate aftermarth of a nuclear war. That was seriously grim viewing, and by the end everyone was dead or dying from radiation sickness. I can't remember what it was called, but it had Steven Guttenburg and Charles Napier in it. No film I have seen has ever depressed me more.
Have to agree with Night of the Living Dead after watching it again.
Not quite sure what you mean by harshest exactly but if I understand you right the first two names that came to my mind were The Wicker Man and Don't Look Now.
Are you thinking of Testament, Muttley76?