The more I read into this, the more I believe that Putin may not have orchestrated the assassination...at least directly. Nemtsov was definitely one of Kadyrov's enemies, and Kadyrov is known for butchering and intimidating his rivals, even ones that reside outside of Chechnya. Just check out this except from his Wikipedia page:
Kadyrov has been personally implicated in several instances of torture and murder. A number of Chechens opposed to Kadyrov have been assassinated abroad, and several witnesses (including Artur Kurmakaev and Ruslan Khalidov) report the existence of a 300-name "Murder List". Chechens who have been murdered, where Kadyrov's involvement is suspected, include Movladi Baisarov and Ruslan Yamadaev (both Moscow); Sulim Yamadaev (Dubai); Gazhi Edilsutanov, Islam Dzahnibekov, Ali Osaev (Istanbul); and Umar Israilov (Vienna).
He's even carried out hits in front of the Kremlin before!
A mutinied commander, Movladi Baisarov, said that Kadyrov "acts like a medieval tyrant. If someone tells the truth about what is going on, it's like signing one's own death warrant. Ramzan is a law unto himself. He can do anything he likes. He can take any woman and do whatever he pleases with her. (...) Ramzan acts with total impunity. I know of many people executed on his express orders and I know exactly where they were buried". On 18 November 2006, Baisarov was killed in an ambush by members of Kadyrov's police on Moscow's Leninsky Prospekt, only a few hundred meters from the Kremlin.
There's a lot more about Kadyrov's predisposition to violence in the article that I linked above. It's totally plausible that he could've ordered the hit on Nemtsov.
That being said, Kadyrov is one of Putin's lapdogs, so he also could've been working at Putin's behest.