Stalins road to terror?

3 (b)
?'But whatever the immediate cause or the deeper motivations of Nikolaev's shot, to thinking Communists it seemed a symbol of the desperations under the policed surface of their country's life. Each of us knew the bitterness and the despair in his own heart. Sometimes we dared share our doubts in twos and threes. It took the murder of one of Stalin's closest associates to make us conscious that our private griefs were part of the great subterranean river of discontent flowing through the heart of a vast nation.'

Viktor Kravchenko, I Chose Freedom (1947)

Is this in reference to the start of Stalin's agressive policy on rivals and opposition? its a documentary commentry im trying to write,but struggling on this extract, i need some help and info,please can anyone help?