> It was used on Alexander Litvinenko because the Russians wanted to show that they were able and willing to take the life of former agents that attacked their interests.

Or maybe not. It is very well known that Russian security services brutally murdered Litvinenko, but there is remarkably little evidence. The only evidence, in fact, is that "everyone knows it". What if new data will make you reconsider this well known wisdom?

> What motivates the use of a radioactive element that very few possess?

It is very widely used and easily available.