This isn't so much a theory, but more of an observation.
JKR had said in a interview that she didn't believe that the books were getting darker. She said all the books were equally dark and that the 6th book was just as bad as the 1st, with Quirrel and Voldemort sharing the same body. I think I would have to disagree here. I think that books are, indeed, darker as the story progresses. I think that the first few books were almost like a fairytale and established the story and the essential Good vs. Evil scenario. There was apparent scary and dark parts of the first few books, but I think as the series progressed, the story just became so much darker and deeper.
I think as we are able to further connect with the characters and learn about them and find out what makes them tick and breathe and live we are able to better empathize with the characters. I think it is these feelings which cause the books to become darker. While I'm sure we could all empathize with Harry from the very beginning, it seems that bad situations just keep getting thrown his way and we get to experience it alongside him, which gives us a greater compassion for his situation. This compassion is what makes the books seem darker, in my opinion. Once we start to feel for and with the characters we become emotionally involved and wrapped up in whatever JKR writes.
So I think the last book will be the darkest of them all, obviously.
Does anybody agree?