Moscow is currently grumbling that their ambassador was put at risk because for ten years the Turks have not allowed them to send members of the Zaslon special operations group to Ankara to provide security. This is such a red herring.
Zaslon is part of the SVR – the Foreign Intelligence Service – and while it sometimes provides some diplomatic security in very extreme cases (as in Damascus, for example), it is essentially an intelligence/sabotage/assassination force. No wonder Ankara didn’t want them there, and in any case it would have been a colossal waste, akin to using the SAS or CIA Special Ops Group as permanent bodyguards. Most security for diplomats in anything short of a war zone (and an art gallery in Ankara is hardly that) is provided by locally-engaged private security guards.
If the Russians really want to ask what went wrong, they should start with explaining why, if for a decade they have thought Turkey such a dangerous posting it needed Spetsnaz protection, they did not hire any security themselves, like their US and indeed UK counterparts? Or maybe they should be chatting with their good friend Erdogan why on-duty Turkish police were not on hand to deal with their wayward off-duty comrade-turned-killer?