1. It was Putin in Russia with a hit squad
Support: Eliminates a rival. Shows his strength. Muah-ah-ah. Along a different path, what is Putin did call the hit on Nemtsov but to frame the Americans? Set up a bunch of patsies, hit a rival and then arrest patsies, reveal it was American meddling, and then the world is wondering how much more insane the USG is growing? Building a separate world system? This becomes a marketing moment.
Drawbacks: Nemtsov's party had 5% favorability in a recent poll. Not 50%, 5%; what a rival. Every Russian ex-pat in the American press will write essays denouncing Vlad. Wait that's already happened since 2012. This fuels opposition, which was having a march 48 hours later. Russians killed a defector with an umbrella decades ago, and have been accused of polonium poising, killing people at their apartments, etc. during Putin's rule. Public execution, leaving behind shell casings" in front of CCTV cameras seems amateur.
2. It was the USG to frame up Putin
Support: Suddenly the opposition protest has a martyr. Nemtsov was not even useful for USG purposes anymore, as his time had passed. This protest has been planned for months and calls for 100,000 were made by every dissident. Khodorkovsky mentioned this as he supported these marches as Navalny marches in just about every interview he did in the winter. You know how many Russians were aware of the planned march? 18%. The financial attack on Russia failed, and it failed to give the population a reason to protest and the US media a way to spin protests as organic and because of the currency crisis. America, including George Soros, has had an eye on destabilizing their Russian rival. The US ambassador to Russia who made an appearance at the public memorial is the same US ambassador who was present in Ukraine during their putsch, and looky-loo, he was also the US ambassador to Georgia during the Georgian brouhaha of 2008. Not just Tefft, but Anatoly Chubais visited the memorial. Chubais was the harvard and US Treasury inside man who allowed us to economically rape Russia in the '90s. Chubais was rewarded for his good work with a position on the Council on Foreign Relations. Ahem, he sits there under his '90s US Treasury buddy Robert Rubin.
Drawbacks: If this were traced back to the US, it would give America the golden shovel for digging a further hole geopolitically. China would be aware considering the Hong Kong protests, and possibly hug Russia closer.
3. It was a move by Ukrainians to frame up Putin
Support: See above for why the US would do it, but add in the recent military and diplomatic whupping they have received. They need to keep American involvement in their sink into the abyss.
Drawbacks: If traced back to them, they lose all support from Western nations except maybe the NY Times readership. The Ukranians cannot even field an army, so pulling off a hit seems like a reach.
4. It was elements within the Russian power hierarchy itching to depose Putin
Support: If traced back to Russians and men part of the security forces, it would be easy to frame up Putin and turn public opinion on him as well as demonize him further internationally. Sacrifice some pawns to take down Putin, and set the pawns' families up. Putin's network and party are firmly in power, but could a man within the network unseat him but keep the web intact? Possible. Stalin had a supreme cult of personality, but Beria and Kruschev both thought they could take over and continue the system after his death.
Drawback: If traced back to top, they would be dealt with, and who then would try something like this in the future?
5. It was Russian Muslims paid by the Saudis to frame up Putin
Support: Muslim identification crosses national boundaries. The Saudis have threatened Russia before and are engaged in oil wars right now with them over Syria. Saudis would actually do something for USG instead of always pushing to "fight the Iranians to the last American".
Drawbacks: The operation seemed smooth for a Muslim group, so it would be Iranians instead of normal idiot jihadis. Jihadis LOVE to take credit for anything, and the entire world would doubt they'd keep their mouths shut about this.
6. It was an element of Mossad to frame up Putin
Support: This was pretty smooth, and they are incredibly gifted at their work. There are enough Israelis of Russian extraction to allow blending in and development of such an operation. Might be the same wing of the Israeli government that is anti-Netanyahu and linked closer with their American cousins. How much coordination between CIA-Mossad-US State Department?
Drawbacks: Do they really have a horse in this? Aren't things warming a bit between Israel and Russia?
7. Nemtsov had enemies who took him out and it was a convenient timing
Support: What figure at the top of any political system is not a bit corrupt and has not made enemies? This was a smooth operation but not much different than a mob hit in execution. Current geopolitical tension can hide you as a suspect. Mob capos can rub out the shooters and eliminate all traces, which makes things harder for the authorities. The authorities need to solve it to erase the cloud of suspicion like how the West tried to pin the shot down Malaysian airliner on Russia for months.
Drawbacks: None. Even if caught, you could claim political involvement, which smears Putin's regime. They'll find patsies to help you out.
This is not Kirov. Putin is not Stalin, but if that is the new analogy, then maybe Hitler has become too passe. Cui bono? Nothing for Putin unless he's trying to frame the Americans. While the upside is high for the Americans, the drawback of exposure would be huge, as would be the danger to any Russian involved with the shooting. Ukrainians, Saudis, and Russian power players all have motivations but they all come with crushing downsides (except the Saudis because of Uncle Sam). Nemtsov had enemies. Could be as simple as that with perfect timing for an international spectacle. We will probably not find out who ordered the shooting and even if we did, who are we going to believe... Putin or America's media?
Order I'm buying (most likely to least likely): 2, 7, 4, 1, 5, 3, 6