Interesting discussion about a tragic situation. I don't think there is any one answer, but you all have touched on a number of valid points. Trying to figure out what has changed to cause this epidemic, I think a lot of things have changed, and some bad things of the past have continued, creating a "perfect storm" leading to gun violence by kids.
We've always had stupid, uncaring, inept, overburdened, or cruel parents. This is not new.
We've always had some kids who were unsupervised, more so today.
We've always had violent games involving shooting. Toy guns were more prevalent in the past than they are now.
Violent video games are newer, but there have always been violent arcade games.
There is probably an element of copycat crimes in all this.
Probably the only way to prevent any more of these school shootings is to have one well-guarded entrance to each school, with the same kind of security checks used in many other public buldings. (Why is it easier for an armed person to walk into a school than to walk into a sports venue, a rock concert, a museum, a government building? Are the children considered less important?)