Litvinenko's poisoning in London with Polonium was a not dissimalar MO. One must ask cui bono, surely?
Wait. You believe Putin would poison someone with Polonium in the UK while having him stabbed by a random hobo in a botched 'hit 'n run' would have the same effect? Do you think Putin is retarded? Emotional? He has always seemed the most intelligent and emotionally composed of all world leaders so that seems rather silly.
I think you have been drinking the western cool-aid a bit to much. What's next? You also think Brexit is bad and that there's any evidence for human caused global warming?