Banning a party isn't just a legal question, it's a practical one. Look what happens when you criminalize a political movement with real working-class support: you drive it underground, you radicalize its fringe, and you hand it a martyr's narrative. That doesn't make Greece safer, it makes the polarization worse. Even if we accept every grim detail about the civil war, and I don't doubt many of those things happened, the right response isn't to outlaw communists, it's to let them stand for election and lose on the merits.
And look at the ethics here. If we condemn the communists for treating their opponents as enemies to be destroyed, banning them would do exactly the same thing, just in reverse. That's not justice, that's revenge. A democratic state has to be better than that. I'd rather judge them by what they do today, in the open, than repeat the very cycle of persecution they once used.