It's interesting you mention this article, as I was reading the same one just a few hours ago. I can tell this is going to be quite an interesting thread already!
Quotes like "It's a video instruction on how to murder somebody, it just shows how you kill people and what weapons you use." seem far fetched, personally. Anyone knows that picking up a gun or inserting a knife through someone would cause more harm than good. Just because a game features plenty of gore and guns doesn't mean that someone is going to go out and enact the previous night’s gameplay.
I do agree however, that there have no doubt been cases where someone has hurt/killed someone from a game/film that that have seen before. But surely that person must have had it in them to start off with, rather than just "copying" the film. I am equally sure lives have been saved from something seen on TV/Game.
If people banned anything that could harm someone, there would be nothing left in this world. Out of 1 billion computer gamers, perhaps 1 a year will do something silly seen in a game (completely made up statistic!). There are more people killed from mis-information, sports, freak accidents etc... This is perhaps taking this to the extreme, but there are "anti" anything groups out there.
Of course, the age limits do seem sensible, even if not properly enforced. I can imagine a 10 year old playing Doom III being quite disturbed by it! A 21 year old should have the judgment to know that this is just a game, and separate from reality.
This seems like a very tragic case, but strange that a "friend" murdered him because of a game. The killer seems very susceptible to outside stimulus, but that does not necessarily mean ban anything that promotes anything contrary to the norm.