Children learn through example. You can talk to them until you're blue in the face, but if you show them, they'll get it.
Those recent shooters mimiced a behavior they've seen somewhere. Did they see their parents shooting people up? Probably not. Their neighbors? Probably not. Their close friends? I doubt it.
Where could these kids have seen this so often they think nothing of doing it themselves?
In order to actually act out what they've seen, they must have seen it often. Where could they have seen this type of thing often enough to be destesitized to it?
Hollywood, games, rap music. It's all there.