Well, this is quite interesting.
I’ve seen one of those Harry Potter spoiler t-shirts, but it was for Half-Blood Prince when the book came out.
This seems like a very interesting subject. This, along with the Milgram Experiment, shows a glimpse of how deep the darker nature of human beings can go. The validity of both experiments are disputed, but it really is a point worth noting.
More information about this phenomenon can be found from the happenings regarding the My Lai Massacre during the Vietnam War.
Also, check these videos.
Stanford Prison Experiment (Documentary): [part 1] [part 2] [part 3]
Milgram Experiment: [video]
Also, modern day replications: [Milgram Re-enactment]