some want to say it started with Rock and Roll… the moral decline of the world. But looking through history, there have always been things that "caused" moral decline, for example in the 20s and 30s it was jazz and swing dancing and alcohol. But there is so much more to the story than that. After Rock became more mainstream, the sights were set to other things, in the 80s it was Dungeons and Dragons and that it "caused" suicides, in the 90s it was cartoons and horror movies "causing" violence, and now in the 2000s its violence in video games and that it is the "cause" of violence.
These items in and of themselves are not the "cause" of anything.
The "cause" is not the games, the music, or the gadgets; the problem is "parents" have become so detached from their children they (the children) have no understanding of right and wrong anymore. The parents look for things to blame for their failings (kind of like Christians blaming politicians for abortions). Parents give their kids "stuff" to shut them up for a while. When I was growing up it was rare to find a kid with a separate phone in his room. Most houses had the phone and television in a central location, and if the children received any outside stimulus the parents were nearby to monitor the situation. Now they have cell phones, TVs, and internet access in their rooms, parents don't have a clue who their kids are talking to, what they are listening to or what they are watching. Kids want to learn and since their parents don't interact with their kids they learn their behavior from others.
You my not like the guy, but Marilyn Manson said it best "The message that I'm sending out to them is raise your kids better or I'll be raising them for you."