It happened so unbelievably fast. One minute, Dawn and the others are safe...and the next she's gone. I think Buffy just couldn't handle any of it. Giles was hurt, Tara was crazed, her mom was dead, and now her sister was gone. It all just happened way too fast. She blamed herself. Not for what happened in the gas station, but for giving up...for that one second in the Magic Box. It kind of reminds me of this one episode of Grey's Anatomy that I randomly watched...Meredith is drowning and she's trying to fight/swim, and then for an instant she just lets go. All of this horrible crap has happened recently and she lets go for just a second, but it's enough, and she drowns. When Buffy thought for a second that she was going to lose, she gave up. That's why she was convinced that she had "killed her sister". It only takes a moment to lose it all.
Ugh, that is so good, in terms of the writing in that episode. That "moment" could apply to Buffy giving up in the Magic Box, for a second, and it could also apply to Dawn being taken in an instant.
I'm not sure if I explained that right, but it makes sense in my head! 😉