Character changing too much?
OK...I can buy that, but looking at the set of circumstances, Harry's had to grow up a lot in a short amount of time, arguably the most in the last three years, considering there's been a major death in like June of every year since POA, and if you count POA, nearly two more (Sirius with the Dementors and Buckbeak's date with the executioner's axe). I can buy Harry aging a *lot* from about March of Year 5 up to now. Imagine if the two men closest to family you have are suddenly dead within a year of each other, then imagine how you'd age psychologically.
Trashy and angsty? OK...I can buy that maybe a book-and-a-half ago, kinda when I was talking about above...about March/April of OotP, but since then?
Hardly. He's had to grow up fast, and when that happens to people in real life, it's not necessarily pretty...but why describe him as trashy and angsty?
As for Kismet's hypothesis, I'm with it mostly. I think the potion was *killing* Dumbledore, but Snape's Avada Kevadra was a mercy blow, if there can be such an animal when talking about "Unforgivable Curses." I think that it's highly possible that Snape is under an Unbreakable Vow with Dumbledore. I know that a lot of people aren't buying that because of "what kind of person Dumbledore was," but let's say that Dumbledore did it waaaay back right after the Potters were killed. According to text, the only other person that heard the prophesy was the "bartender" at the Hog's Head. That would be Aberforth Dumbledore, and what perfect way to introduce him in book 7 as not only the brother of the greatest wizard of the time, but also the other member of the OotP that JKR mentions that we'll get to 'officially' meet in book 7, *AND* just happens to be the one to clear up whose side Snape is on...but others will say how?
Easy. He was the Bonder of the Unbreakable Vow between Snape and his brother. Ladies and gentlemen, a revelation.
Besides, if Snape's working for LV, then we have two of the same villain. Nah...JKR wouldn't do that to us. Voldemort has to be the bad guy. Snape is the classic double agent, but how we get to that point in the book will be very near the end.
As for Dumbledore's death itself, I think is was done with witnesses present for a reason. I'm not going out there on that limb and say that Dumbledore isn't dead...yet, but it sure worked out, didn't it. Death Eaters and Harry and Malfoy all around, the way Dumbledore's body was blown off the parapet, like Kismet said, and the parts during the chase when Harry calls Snape a coward and the reaction to that, and the fact that Snape saved Harry from a Cruciatus Curse, and possibly from death. What benefit could that serve unless Snape is still bound to Dumbledore's Vow.
Just some thoughts....
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