Fundamentally, I believe we disagree on Communism itself. The USSR was honestly pursuing Marxist Communism, and was not a betrayal of such values. However, you believe Communism to be more pure, more anarchic, and thus see the USSR as a betrayal of those values.
I believe we should judge the USSR along Marxist lines, rather than Anarcho-Communist lines, as the USSR never claimed to be Anarcho-Communist (though they revered Kropotkin and named the largest train station, Kropotkinskaya, after him).