Damnit, Ellie! If you want to learn more about how Naughty Dog created Ellie's AI for The Last of Us, you should definitely watch this video from the GDC Vault.
Though technically only a spoiler in that it's not what happens, though responses may contain spoilers, assuming there are any (responses I mean).
I couldn't believe it when someone told me Joel doesn't kill Ellie. I was 100% expecting that to be how The Last of Us would end, I thought that Joel would kill Ellie as recompense for the lost of his daughter at human hands, a sort of ultimate act of spite from a man who has lost all faith that humanity deserves the last hope that Ellie represents. I reckon that would have been really thought provoking, fitting and chilling end to the game. It's just not how I would have wrote it I guess. The end it has is alright too, I just wanted it to take that one final step.