No offense intended - but I find placing blame on anyone but the perpetrators and incidentally, their parenting is a cop-out. We are all looking to point fingers and to have quick solutions. But we make games, music, movies and TV (and guns, and sex, and other entertainment) all scapegoats. It doesn't solve the problem and only serves to mask the true source. Inevitably, limits are placed on adult access to legal endeavors when politicians looking to be seen to be doing ANYTHING start cracking down.
Sadly enough, those politicians are doing what the public really wants. In many instances, the public is as misguided with our societal stewardship as those parents are with their troubled children.
Even so, blame lies with the ones causing trouble. They should be made to pay. I'd say the 3 who attacked may have been looking to do more than simply mug our poor friend. I hope they are caught and punished.
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Experience is a hard teacher because she gives the test first, the lesson afterwards.