The term tankie has been vulgarized, but it has a specific meaning. A tankie is an orthodox communist who puts the “dictatorship of proletariat” above any morality or any other interest, foreign or domestic. That’s how a tankie brushes away any genocide, any crime, because no value or entity is too important to stand in the way of their communist utopia, even human rights. In this sense, this group is very close in practice to nazis, you just have to replace “proletariat” with “nation” and for that abstraction anything is allowed: internal repression, invading your neighbours and allies.
But not all “communists”/socialists are like that and many mean well (e.g. Gorbachev, some anarchists like Chomsky or “democratic-communist” Lula, etc), but a good few follow tankies unquestioningly without realising the underlying inhumanity dressed as theory.