Agreed that the colonels and the run-up to the coup--king's dismissal of Papandreou, king's initial compliance with the coup--certainly didn't make anything any better. And the Civil War was overwhelmingly brutal: better than Guatemala, but probably worse than El Salvador...
I'm not sure why you say that we don't know what hte alternative would have looked like had Greece had a really-existing-socialist government for the entire post-WWII period. We have lots of examples of such governments in Eastern Europe, and know a lot about what they look like...
Brad DeLong