I am reading a book that states the hero "knew himself to be among the dead". Does this means that he considered himself as good as dead, or that he understood that all the people around him were dead?
Generally, what does "among the dead" means?
As far as I know, there is no idiomatic usage of "among the dead". It appears to be used in both of the contexts you've provided. In your case, it's that everyone around him is literally dead.
...those hideous nights when we toss and turn in fever and pain, when we lie, like living men among the dead, staring out into the dark hours that drift so slowly between us and the light. - Idle Thoughts of an Idle Fellow, Jerome, Jerome K.
If they do not square accounts with me about my cows, I will go down to Hades and shine there among the dead. - Homer, The Odyssey
She did not know any of the names listed among the dead. - Cambridge Dictionary
Naturally, the villagers show up and drive the monster away, and he goes to a graveyard to find solitude among the dead. - a summation of Bride of Frankenstein
There are "dead" idioms, such as, dead as a doornail, dead ringer and over my dead body, but not among the dead.
I'm not so sure Susan...if the protagonist really did walk into a restaurant and find that it's been robbed, where's the suggestion that he's literally surrounded by dead people? And if he did walk into the restaurant and find, as you suggest, everyone dead, it would be rather clumsy of the narrator if the intention was to repeat that obvious point: 'He knew himself to be among the dead'. Duh.
I think some subtlety is being missed. Does the protagonist feel terribly afraid and unsafe? Is it that the protagonist feels threatened or targeted, so that he feels as good as dead? Or (and this is a stretch), does the protagonist feel 'something die in him' on being faced with such devastation?
I don't know, but I think a deeper reading is required.
The main character in Daniel Akst's book, Burl Bennett, is an obituary writer. So using the phrase 'be among the dead' at this awkward moment is perhaps more a reflection of his professional capacity. Seeing the dead he might of course also have been fearing for his life, since he was co-owner of the restaurant in question.