In the end of season 6, Giles comes back to Sunnydale, and Buffy tells him everything that has happened since he left. Dawn's kleptomania, Willow's going evil, her sleeping with Spike, Xander leaving Anya at the altar, Tara's death.... It seems like in this season, more has happened, mostly bad things, during that year than ever before. Why? who's to blame? Is it Giles for leaving? Is it Buffy's friends who brought her back to life? Is it the Trio who decided to make up troubles in Sunnydale? Who is responsible for the mess this season? So many bad things at once, why did it happen? Is it just bad luck?
I don't think anyone was to blame.
Dawn's stealing:I think Dawn started stealing for a variety of reasons. One, I think it made her invincible, as most teenagers think they are. Stealing made her feel powerful. And in a family where she was the "freaky magical Key", and so many people around her and close to her had superpowers, Dawn needed something to feel special. Stealing did that for her, I think. The other reason I think Dawn stole was because of peer pressure. Her classmates probably had nicer stuff then she had, and more money, and she gave into the pressure. teens that age often lack self-control. They steal to test the limits. Remember the jacket she stole for Buffy? Dawn probobly felt useless because she couldn't buy her sister a present. Also, with teens, stealing is a form of rebellion or a "thrill ride." Dawn has always been a thrill-seeker.
Willow's evil: I think we all know that Willow had been experimenting with how far she could go with magic for a long time, so much so that people who cared for her, such as a Tara, where very concerned. I feel like Evil!Willow was inevitable. Willow had to hit rock-bottom to realize that she has an issue.
Buffy's sleeping with Spike: Well, this was a long time coming, too. These two had always had a "thing" for eachother. Do I believe Buffy would have slept with Spike if she hadn't died? I lean towards no, but I do believe that Buffy has always and still is attracted to her "dark side", and Spike was a way to experiment with that.
Xander leaving Anya at the altar: Well, Xander had expressed fear that he was rushing into this. All that caused this was Xander not being honest with himself about how ready he was for that type of commitment.
Life is responsible for this season's mess. Life throws things at you, and all you can do is deal. Buffy and her friends learned that the hard way.
As one of my clients told me recently;
"The word "unexpected" should be under "Life" in the Dictionary."
The Scoobies, Buffy....no one knew what was going to happen. It just does. It's how you handle it that matters.