I don't find the insider risk idea hard to believe. The primary stuff is still an open wound, and you could also easily imagine someone disgruntled for more prosaic reasons (e.g., passed over for a promotion). The fact is we have very little evidence for any of these ideas, and I, for one, don't find it reasonable to conclude that it is therefore obviously the fault of the Russians, especially given how unhinged and paranoid rhetoric about Russia has gotten.