Which interpretation of the ending do you like best? *Spoilers*
From what I understand, there are three ways to interpret the ending of the movie (and therefore the plot of the movie itself), where Walker just sort of slides into darkness with no resolution concerning the money.
(1)That everything that happens in the movie is "real." Walker is a criminal who, after being left for dead, miraculously survives and plots his revenge. He dissappears at the end of the movie either because (1) he knows there's no money in the suitcase and that he's probably not going to get his $93,000, (2) he realizes that there's always going to be a higher step in hierarchy to the organization and it's not worth the hassle to keep fighting, or (3) he was just waiting for Carter to leave so he can get his money later. This explanation seems to be most popular with the typical viewer.
(2)Walker is mortally wounded at Alcatraz, and as he lies dying in his cell, he concocts an intricate revenge scenario in his head as his dying wish. This explanation would seem to explain all the strange coincidences and incidents of good luck that Walker has during his course of his revenge. Fantasies are not often realistic, and likewise, in your dream scenario everything plays out exactly like you want it to. This could also explain his vanishing at the end: with his short few moments before expiring, Walker unlikely thought about anything beyond just wanting his money back: having seemingly accomplished his goals, and getting a compliment from the crime boss Carter for doing so (another fantasy), he had nothing more to think about.
(3)The most supernatural explanation: Walker did indeed die from his wounds on Alcatraz, but returns as a ghost to exact revenge. Again, this would explain all the coincidences and good luck he faces (in this explanation, it would be because as a ghost, he could show up wherever he wanted). He manages to kill off those responsible for his death, and does so by playing them off against each other. The explanation for his dissappearence this time would simply be him leaving the mortal plane since his work was done.
My problem with the first explanation is that it doesn't explain several of Walker's strange actions throughout the movie.
My problem with the second is that if this is a revenge fantasy, why go about it in such a half-hearted way? Walker doesn't kill anyone throughout the course of the movie: if I was planning my perfect revenge plan, I would want to personally take care of everyone involved.
And my problem with the final one is, if Walker is a ghost, why would he simply vanish without checking to see if he "got his money?" If that's thing that was driving your revenge (remember, thoughout the movie Walker says he wants his money back, he never suggests he is motivated *purely* by revenge) then what was the point of his return from the grave?