Whoever he was, he must play for the Horde, or maybe he was just a bit miffed over the recent Mists of Pandaria expansion. A hacker recently found a way to make his level one character fly around the city of Stormwind, the Alliance capitol, killing anyone and everyone that got too close to him.
No one has claimed responsibility online, and Blizzard has yet to release any sort of official statement, it seems they’ve been tied up with another problem leaving players stuck at the connecting screen. Are the two related? We don’t know yet, but if someone can go around slaughtering like this, either they found some major vulnerability, or it’s time to break out the Sword of a Thousand Truths.
Blizzard has already begun rolling restarts of the live servers to fix the connection issue. Whether or not they manage to patch the hole allowing players to go on killing sprees remains to be seen, but some rumors attest that it may be related to players gaining access to GM commands. Check out this video and see the carnage for yourself.