Or Serbia, Belarus, Armenia, the Central Asian former SSR's, a large part of the Ukraine.... so tell me this, if Estonia should deny citizenship to people who can only speak Russian, should Ukraine do the same? Its 100 % comparable, now stand on your principals. should Ukraine do the same?
This is not 100% comparable, both historical background and legal background are vastly different. Plus, it is not possible to speak Russian only. I have never learned a word of Ukrainian, yet I could understand roughly 2/3 of Ukrainian texts due to their similarity with Russian, when I visited Kiev and Simferopol this summer.
Next, Estonia does not deny citizenship to anybody.
People who had been Estonian citizens in 1939 and their children were automatically recognized as Estonian citizens in 1991. Including Russians, few as they were back then. All the rest were considered illegal immigrants (as they are according to international law) who have to adhere to few perfectly reasonable demands to get their citizenship. From 1991 till 2008, roughly 150 000 Russian-speaking people have acquired Estonian citizenship. Among the rest, many do not want to, because then they'd lose option to get to Russia without visa.