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SPOILER YO!
I like to think that is her recognition he has got where he wanted to be. Which is a fucking scary and accurate depiction of suicide in my experience.
You couldn't end a film with a literal depiction of his death. You just can't. It would be borderline irresponsible letting people leave the screen in that state, not to mention commercial... er... suicide. It also wouldn't add anything – we're all adults and can read between the lines... except @EB obvs. :P
SPOILER ALERT
It's a film about a man killing himself. It's not ambiguous, it's just not graphic.