Alternative was given by the poster I was answering to: a false flag. Who has anything to gain? Basically anybody, who needs any reason to blame Russia for everything, which is a very popular thing to do for the last 10 years at least, and getting more and more popular lately. Intimidating the opposition? I already said why I don't buy it. There is no real opposition to intimidate. Navalny does what he does for the last 10 years, he didn't become any more dangerous — less dangerous, in fact, since novelty wore off and he isn't perceived very seriously anymore. BBC may call him "opposition leader", but most of the Russian people who don't like Putin (and there is a lot of them) don't really see him as a leader, more like "yet another clown".
Let me put it this way. There is a "perceived value" and a "real value" of assassination. So, if you want to send a message, "real value" is metric of people you want to get a message getting this particular message. Like if you are GRU/CIA/Medellin cartel (basically the same things) and you want every cartel member to know that traitors will be punished, you want it to be relatable for the other cartel members and gruesome. You don't really want it to be very much high-profile (which it must be to some degree if you want it to be gruesome, but only as a side effect).
"Perceived value" is what is "real" to complete outsiders unaware of situation. It is what will cause the resonance in BBC and among the people you really don't give a fuck about getting the message (and in case of Russia govt. — don't want to get the message: Russia absolutely isn't trying to tell the world there is a dictatorship, it wants for the outsiders to get exactly the opposite message, that there is "real competition", i.e. people like Navalny that are against the regime, but just don't have that much of a support).
So, to summarize, if you are Medellin cartel sending message to its members: maximize "real" minimizing "perceived". If you want to set up Medellin cartel: you maximize "perceived" and don't give a fuck about "real".
Now, what is real/perceived ratio in the case of Navalny? As I already said, this is my personal (pretty humble in this case, and not particularly strong) opinion, but my estimation is: very, very low. I don't know of any "people just like Navalny" (so it will be relatable), that are actually dangerous to the regime and need to get "a message". On the other hand, "perceived value" is very high: in fact, the moment Navalny gets to a hospital, everyone automatically assumes it is an assassination by the Russian govt, since he is such a prominent "opposition leader".
So, if I was GRU, I wouldn't want Navalny to be killed in an obvious way. If I was a CIA, I most likely would.