There's a lot more you can do with $10,000,000 (yes, ten million) dollars than to kill a spy. I'm sure that killing a spy can cost you a whole lot less! What would you do with $10 million? I sure wouldn't kill anyone!
It seems that Alexander Litvinenko's killers used an absurd amount of plutonium to kill the guy. Geesh, either they wanted him very dead or they wanted to kill everyone around him.
Read about it here.