Where does it come from inside a civilised society? From without, it comes from ruthless ideologies, we know that. Harder to understand from afar why it happens in a civilised society, and why it's perpetrated by right minded people. Where do I feel safer? Should be London, but somehow the intimacy of being stabbed with a knife is more real and threatening than the distance and gutlessness of being shot in New York. Is there an inherent violent trait in men, that has not evolved to the same degree as other traits, and is more easily triggered in some than others? Of course our societal environment, and our unique experience of it, and more importantly our reaction and response to it, must play a role.