"Every time I call (my mother)," said Russian opposition leader Boris Nemtsov recently, "she gives me a talking-to: 'When will you stop being rude about Putin? He'll kill you.' "

Now Nemtsov is dead: four bullets in the chest as he was walking home in Moscow with his girlfriend on Friday night. The protest march against Putin and the war in Ukraine that he was planning to lead on Sunday became a memorial march instead.

So, two questions. Did President Vladimir Putin order the assassination? And if he didn't, then who did, and why?