It has always been unknown where atrocities will strike. Both armed angry individuals and terrorists with political agenda have popped up in many different countries over many years in many centuries. Remember, remember Guy Fawkes and the Catesby plot. In the UK Special Branch was founded in the nineteenth century to deal with Irish political terrorism and when that had been scotched, it had to deal with anarchists from Peter the Painter onwards. This is not a 21st-century phenomenon; it's just that we have 24/7 news and rather more sophisticated and lethal weaponry than Gavrilo Princip who managed to managed to ignite the Great War – at least, he did, the second time round.
I'm not being flippant. I lived in London during the time when our bags were searched every time we went into a shop, cinema or theatre. I was a short walk away from a bomb going off in Oxford Street anda short ride from one going off at Victoria Station. I was travelling in a car to a trade fair in Frankfurt when Hans Martin Schleyer (not sure of the spelling) had been kidnapped (and, as it turned out, killed) by Baader Meinhof. Of course it is frightening; it has always been frightening for those living through such times. However, it is important not to get things out of proportion and see a bigger evil than the smaller evil that has happened.