Does Jack actually know who he is?
After watching the movie and reading this board with people arguing whether Jack is Cain or a fallen angel, or somehow both, it occurred to me that perhaps even Jack does not know.
Remember he is thousands of years old. He is definitely traumatized by the weight of his memories. Remember, without feeding on blood, he is literally almost deafened by the constant soundtrack of his memories. He does heal from being shot, stabbed, Maced, and drowned, but he seems to feel the pain of all his injuries.
Watch him walk, I have an elderly neighbour who was in a very bad car accident years ago, and it hit me that it was exactly the same tentative, stiff walk. Yet, Jack CAN move blinding fast and smooth. It seems if the stiff walk was more a mental response to all the years and pain he has endured than the actual state of his body. Pain, love, loss, endless change, all of life's experience century after century maybe just overwhelming.
Could it be that, after THOUSANDS of years, Jack can't actually remember who he is, that he has internalized the stories and myths that he has come across to explain himself to himself?
There IS definitely something supernatural going on. After all, Jack can't die, or even permanently be injured. The Man in Black who watches him, and apparently has done so for at least centuries, is supernatural. But perhaps Jack himself does not even know who the Man is?