Anyway, to the point: this wasn't simply an assassination. There are any number of poisons out there that would do the job painfully well but much more rapidly, and without the same scope for a diplomatic incident. Likewise, a bullet to the back of the head would have worked just as well (as witness the assassination of Anna Politkovskaya).Well yes, the capacity demonstrated by some shadowy somebody who may or may not be in Russia is scary -- but no more so than the shadowy somebody who took the equally lethal weapons grade anthrax sporesfrom a U.S. weapons facility and infiltrated the US postal system with the stuff. Even more people died in that case. Nobody was caught in that case either. Remember?
What this is, is a warning: "we have the capability to detonate a dirty bomb in central London any time we feel like it, so don't fuck with us". (Just take Polonium and add a little TNT.)
Let's face it, current and past superpowers have long had the capability to kill damn near everybody on the planet and it seems that every once in awhile some individual affiliated with one or the other just can't resist taking the toys out and playing with them, just a little.