The story repeated in that last comment, the one about the father that secretly set up the camera to capture his son's out-of-control behavior, is the one you most often see tacked onto the video (athough another variation is that he himself left it on, which is the one PC World went with in their coverage). Both of these stories appear to have been created purely to make the video scarier.
After all, it would be supremely uninteresting to find out, for instance, that the kid has made many videos, all of which feature him playing some goofy exaggerated character. Well, too bad, because that's the truth.
The kid (often referred to as "Leopold" but his name has generally been kept out of the press since he's a minor) made the video in the wake of Germany's own violent video games controversy, when lawmakers were campaigning to restrict their sale. Leopold did his impression of the politician's worst nightmare, slamming around the keyboard and laughing maniacally as he killed opponents.
Germany's Focus TV saw the clip, and got permission from the kid to use it on the air. Leopold and his father then watched as the TV show manufactured a backstory--the one about the father secretly recording his bloodthirsty, ADHD son--which then followed the clip as it swept across the web. Holy crap, maybe we owe Fox News an apology.
Once the clip reached English-speaking audiences, even more of the context was stripped away since all they got was incoherent, enraged babble. If you get a correctly subtitled version, on the other hand, and watch it all the way through, the joke becomes fairly obvious (See the bit with the lost Escape key and him telling his opponent to eat his "melted s***").
