We don't know who gave the order to murder Boris Nemtzov.
Here is what we do know:
1. In the past Russian government didn't shy away from breaking the law to compromise Nemtzov. FSB recorded his cellphone conversations and published extracts containing swearing.
2. When Putin murders out of vengeance, he likes to leave a clue that it was him so that he can present it as a lesson to others. Po-210 was used to murder Litvinenko.
Nemtzov was murdered right next to the Kremlin. That would have been very hard for someone without access to Nemtzov's communications and knowledge of his route; ordinary criminals usually kill targets on the porch of the target's dwelling.
3. For that matter, professionals wouldn't select the constantly monitored center of Moscow, unless they had reasons to believe they won't be punished.
The murderer of Litvinenko is an MP in the Russian Duma.