Conversation so far:
How was Alexander Litvinenko poisoned?
His illness was later attributed to poisoning with radionuclide polonium-210.
who poisoned him?
In interviews, Litvinenko stated that he met with two former KGB agents early on the day he fell ill - Dmitry Kovtun and Andrei Lugovoy.

Question: why was he poisoned?
Self-contained rewrite: why was Alexander Litvinenko poisoned?