Direct experience of murder

The other point I would make: Even with all the murders in the United States since the Kennedys were killed, very few people have experienced murder directly. 

That's a huge cultural shift.  In Leviathan, for example, Hobbes argues that we seek to avoid shame and that the most shameful thing there is is to die at the hands of another. He wrote that for an audience who all would have known at least one person who died a violent death at the hands of another.

There are lots of movies with high body counts from murder or war but comparatively few with high body counts from highway accidents. The difference is that we all already knew of someone who died or will someday live to experience someone we know die in a car crash, so we demand that these things be treated with moral seriousness.