I love this because it underscores that no matter who is or isn't there with us when we die, we die alone, with that one last thought. Reminds me of an episode of 'Dead Like Me', where it's revealed that the last thought of everyone who dies is saved and cataloged. It was touching to hear some of them--some so profound, some so mundane, but all of them personal and fitting and, well, last.
I wonder what Snape's last thought would be. Would it be about Harry, or Lily, or the Dark Lord? Or would it be something like 'I'm thirsty.' Intriguing. And very brave of you to paint such a realistic and thought-provoking scene.