It's all about shock value. Haneke's intro for this one is to desensitize you for the rest. What happened with the teenagers is nothing compared to what we see first because the first one is real (and we know it's real), what happened with the teenagers is fake and off-camera.
The discussion happens from this point. Why aren't we more shocked? Why is Benny so insensitive to what he did? It's because of his hobby. He doesn't see it as real, it's all from his camera point of view, like everything he saw before. It's just digital or analog information put on screen. He sees himself as a viewer and a camera at the same time. He is dehumanized, he is a machine, that gun was part of him as well. And like all Haneke, it's shocking.
The discussion happens from this point. Why aren't we more shocked? Why is Benny so insensitive to what he did? It's because of his hobby. He doesn't see it as real, it's all from his camera point of view, like everything he saw before. It's just digital or analog information put on screen. He sees himself as a viewer and a camera at the same time. He is dehumanized, he is a machine, that gun was part of him as well. And like all Haneke, it's shocking.