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it's never good to make a martyr of an opposition figure
And that idea was probably part the calculation that allowed Navalny to stay alive for so long. But as Russia is edging closer to elections it probably became urgent to set an example for every other aspiring Navalnys out there. Last time around Navalny had just enough influence to make things awkward. Navalny was barred, but Ksenia Schobchak ran for office as the face of the nation's discontent. Not that Ksenia Schobchak and Navalny were the best of friends, they fell out eventually, but Schobchak gained a lot from Navalny's campaign urging people to vote for any party except United Russia.
For sentimental reasons* it will probably be very difficult for Putin to throw Schobchak out of a window, but by killing Navalny he may have managed to get her in line this time around. Without Navalny as a central focus point of discontent and his campaign to get people to vote, Schobchak will have to play along with the system to be heard. She'll probably drop out though.
I read somewhere that it's never good to make a martyr of an opposition figure, however weak it is