'When I were a lad' I used to play Cowboys and Indians and Army. Sticks for guns possibly a plastic rifle and cap guns. No concept at all of what really happened out there. You might get pictures on the TV of the latest conflict in the world, but were spared the graphic detail.
Fast forward to now and children (and adults) are exposed to all sorts of violence on TV, games, internet and that's acknowledging that some form of censorship should be in place. That rubs off on them. I know, I see it in my grandson, although fortunately he has a morality that will hopefully stop him turning the wrong way in later life. There are many however who do not have this morality.
The media sensationalise and glorify violence because it sells. And that manifests itself in many ways like carrying a knife and not being afraid to use it, and, sadly, driving a van at innocent people.