What is it with the Ruskies and poison?
I don't understand this either. Just have someone rubbed out Mob style. Seems cleaner and there is no "rare poison" m.o. to make it look like an act of a government.
Lethal radiation poisening causes a long, slow, painful death. This grotesque form of murder is probably being used as a stark, psychological deterrent to keep other defectors from revealing the dark truths about Putin and Russia.