First of all, I have never approved of and never will co-sign on extra-judicial murders. With that being said, it is always easy to pass judgment from the comfort of our cubicles and living rooms. What we forget is that a government's loyalties lies with its populace and that national security trumps everything. Within this frame of thought, it would be safe to say that Litvinenko may have been deemed a credible threat to the national security apparatus in Russia.
No one has the right to approve of killing someone. There are plenty of monsters out there that need to be taken care of and put away forever. That being said if it was between them and me. I will not be going quietly I would take them with me. I think murder in the right circumstances is in all of us.
Some might argue that no human being has the right to order the murder of another. However, as a spy working for the Russian government, Litvinenko's life was in part controlled by the Russian government, and President Putin. If Litvinenko did something treasonous, perhaps his murder would be justified, but if the order to kill him was based on a personal bias Putin had against him, then his murder was not justified.
I believe that the murder of Litvinenko approved by Putin is an assassination. They organised, planned and executed basic steps to kill him after so many years of opposition between them. I think Putin should be brought up in a certain court to answer to this crime across the nations.