I personally suspect at this stage, his death would turn him into a martyr and the last thing the individuals operating these illegal surveillance programs want is more attention back on what they are doing.
Putin gives him protection because it lets him thumb his nose at the US.
Not so much can't get a chance as the price would be too high. He's under Russian protection. That protection is political far more than tactical. The CIA could probably pick him up in less than an hour if they knew they could do it without pissing off the Russians. It's the "pissing off the Russians" part that's stopping them, not the logistics.
As for whether the CIA would want Snowden dead - I presume it would only have been useful to them pre-leak, and I doubt murder would have been on the cards then anyway. I don't see retribution as being particularly high on the CIA's list of priorities, and the political entities in the US that are keeping Snowden relevant wouldn't appear to gain anything from his death.
Obama said as much in why he pardoned Manning and not Snowden, in that Manning has already gone through the justice system whereas Snowden has not.
In that context, it's very easy to imagine ways to frame the death such that it would prevent him from seeming a martyr. Died in a car accident, strangled during auto-erotic asphyxiation, shot in a drug deal gone bad, depression-related suicide, etc. The right context could be used to discredit him further, in addition to silencing any potential impact by giving speeches or serving as a public figure for a privacy-focused organization in the future (for example).
Of course, you could then respond that it would still spawn conspiracy theories, but the doubt planted would certainly blunt the kind of wide-spread movement (or public outrage) that might otherwise sprout from a bald-faced assassination. Realistically, conspiracy theories will pop up in any situation in which Snowden doesn't die of old age at 95, so this isn't much of a dissuading factor for our hypothetical villains.
We know that state-sponsored assassination is possible, based on the basically-obvious case of Litvinenko. Crucially, we can also never know of any attempt which was successfully reframed, which makes it impossible to determine whether or not these "accidents" exist; there are many, many deaths which have been nominated as candidates, but no conclusive evidence of those that I'm aware of.
So, rather than "would the people in power allow it", I think the following 3 questions are more pertinent:
1 - Are nation-states competent enough to plausibly disguise the attempt?
2 - Which nation-states conduct this kind of behavior?
3 - Who in the intersection of Q1 and Q2 cares enough about Snowden to do it?