a frank answer - i don't know b/c until movingtarget posted about the assassination of nemtsov i had no faintest idea WHO nemtsov was...i looked him up in the wiki and goggled the news.
all i arrived at was that if a political critic was to be removed (keep in mind, i look at things - at the world, in fact - through the prizm of common sense and the practical cause/effect)
it did not make any sense to kill the dude on the red square and, most importantly, a day before the official moscow march he was to head - a march of vlad's opponents.
my sense tells me that only a drunk or a
deranged politician would imagine gaining from electrifying his enemies A DAY BEFORE they were to gather to denounce him.
forgot the latin term for the motivation - cosus belli (?) - does not fit.
otoh, a party interested in flaming the protesters against vlad would have a perfect cosus belli (sp?) my cause/affect mindset tells me.
i simply have no idea and the kgb, after all, may have come up with a scenario no westerner can figure provided the 'mysterious russians' are a real thing