Well, it's easy to assume that Putin's behind this but.... why would he do this? His approval is like 86% now and nothing Nemtsov could have done was going to change that.
Reporters and Politicians are assassinated or tried on trumped up charges fairly regularly in Russia. That's what caps Russia's potential and guarantees they will always be a second-weight power in international politics. They can exert force on their borders and are very powerful there but their reach beyond that is very weak. You can't spread a culture that is closed and fear-locked.
So I guess you're dubious when you heard about Kim Jon iL ' golfing prowess? "After picking up a golf club that day for the very first time in his life, the Dear Leader of the Democratic People's Republic of North Korea fired a 38-under-par round of 34 at Pyongyang. According to the 17 security guards who observed the performance, the score included an amazing 11 aces. Naturally, the event was dutifully reported to the North Korean masses by the state news agency." Seems legit. _