This article says it best, as to what the most likely scenario is: http://news.yahoo.com/blood-near-kremlin-russias-media-fight-back-152448569.html While the murder is so far unsolved, Putin's critics say the 'fifth column' rhetoric has helped to create a climate in which pro-Kremlin hardliners could have felt they were performing a patriotic duty in disposing of a man like Nemtsov. is it most likely that Putin had him killed? not as of yet. But Putin certainly helped create an atmosphere of paranoia and hate, that would allow his more ardent supporters to kill Nemtsov, thinking such a deed would be an act of patriotism against a "traitor".
