For those who don't know:
There is one point of the game that is very frustrating, especially for first-timers, where you have to run across narrow ledges along the side of a huge temple while there are silver knight archers on the rooftop who shoot you with arrows that knock you off the ledge if they hit you.
At the end, you gotta make it past an archer that is blocking you on the ledge, and it is very difficult to do, but once you do, you turn the corner and you are home-free.
What this guy did is invade the player's game, cosplaying as a silver knight archer with the same bow, and when the player turns the corner after a difficult portion of the game thinking he has won, he shoots him and sends him to his death.
This is what Darksouls is all about.