Was it communism that killed people in the Soviet times, or was it authoritarianism? You can have authoritarians no matter your economic structure. They have killed people around the world in the name of communism, capitalism, fascism, race, culture, religion, colonialism, and just general jingoistic control for millennia.
Ask it the other way around. Can you have (Marxist/Leninist) communism without authoritarianism? The answer certainly seems to be no. And since the Marxist/Leninist flavor is what almost everyone thinks of when they say "communism", then one could say that, absent any qualifiers like "voluntary" or "non-Marxist", communism is authoritarian, and did in fact kill all those people.
Was it Leninism that was so deadly in the USSR or Stalinism? Stalin was a much different leader with much different politics and philosophies than Lenin and Trotsky. Don’t confuse the Communist party with communism, just like many Libertarians in the US are not actually libertarian these days.
