The girl that took the photo is just so incredibly lack of empathy and awareness to history and what she did is so tacky and tasteless its incredible.
The fact is, is that Selfies are just a new form of really annoying Narcissism that epitomizes what people hate about Teenagers in America. So obsessed over petty and juvenile things and so into themselves that it makes people cringe. This isn't a new phenomenon, its just so much more easily seen now with Social Media.
The fact that the girl was so obsessed with making sure she took a photo of Auschwitz, not because its such an important place in the history of the world, but because she wanted to brag and show off the fact that she was at Auschwitz. It shows that she has no understanding or perspective of what that place is to the world at large. That's probably a testament to where she is from and the level of education she was exposed to.
She doesn't deserve death threats, shes just ignorant and a little bit narcissistic. I do think that someone needs to sit down with her and explain that what she did was not cool. Though, I don't know how well a teenager would take the idea that they are wrong. But who knows