If you wanted to send a message to your opponents, or keep your current employees in line, then this is a fairly good method, if you have a certain lack of morals. another thing about this is that it is such a bizzare method, the media would have to be actively prevented from reporting on this, so if you wanted to send a message, then this method lets you use the entire media as a megaphone.
Both of the two competing theories could be correct, it depends on how fractured internally the FSB is. he could be a member of one faction, who has got to the situation where another faction is beginning to see him as a threat. The fact that he has left may be part of an obvious "the west"(i know i'm using it again) versus the FSB game. where in his mind and internally to the FSB it could appear to be part of the faction A versus faction B game.
On the other hand have another theory that can't be ruled out, if the man has been planted with false information (that he dosn't know is false) then induced to defect. Then killing him would reinforce the reliability of the information that he has passed on.
We are working with a very limited set of information, one of several groups of people have killed a man with an obscure but horrifying method, Beyond that all is speculation. Life should consist in at least fifty percent pure waste of time, and the rest doing what you please.
It can certainly be a faction fight...
BUt of course, the most intriguing thing here, it is the involvement of Berzoski and the british secret service at the same time. This last item makes me guess that things are much more complciated that pure killing becasue he was learning too much about Anna murder in Moscow.
There must be certainly more than this...I have to close with your last apragraph. "one of several group have killed a man with an obscure and horrifying method".. beyond that.. jsut pure spy novel speculation.
A pleasure I therefore claim to show, not how men think in myths, but how myths operate in men's minds without their being aware of the fact. Levi-Strauss, Claude
You note the involvement of the secret service, and Brezhoski, and if you want to stretch that a bit further you could add an entire cast of assorted spooks and crooks who can be summoned up with motive with no problem whatsoever.
I think I probably prefer your versionof my last paragraph. Life should consist in at least fifty percent pure waste of time, and the rest doing what you please.