He didn't call them terrorists, that was a translation error that inflammatory media outlets didn't bother to correct.
Where do you think the mistranslation happened? Here's the text (in Spanish) from the El Mundo interview. My Spanish isn't great, but it seems quite clear:
(El Mundo) Aunque no haya encuestas, en las calles de Atenas lo que se siente es que la gente cada día que pasa tiene más miedo.
(Varoufakis) Sí, así es. Lo que están haciendo con Grecia tiene un nombre: terrorismo
(El Mundo) ¿De verdad piensa que lo que están haciendo con Grecia es terrorismo?
(Varoufakis) Por supuesto que lo pienso: es terrorismo. ¿Por qué nos han forzado a cerrar los bancos? Para insuflar el miedo en la gente. Y cuando se trata de extender el terror, a ese fenómeno se le llama terrorismo. Pero confío en que el miedo no gane.
(El Mundo) Even though there are no polls, what you feel in Athen's streets is that people is more scared every day.
(Varoufakis) Yes, that's it. What they are doing to Greece has a name: terrorism.
(El Mundo) Do you really think that what they are doing to Greece is terrorism?
(Varoufakis) Of course I think so: it is terrorism. Why did they force us to close our banks? (They did so) To instill fear into people. And when the objective is to spread terror, that phenomenon is called terrorism. But I have faith in that fear won't win.
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My personal stance is that he did indeed call their counterparties terrorists. However, he carefully distinguished them from "violent terriorists" such as ISIS et al. Which kind of terrorists are actually more harmful in the long run... that's a matter for personal opinion.
But the guy have a penchant for not mincing words...
However, if his words are commonly misrepresented, then he should be more careful when speaking publicly. Part of being a politician, and particularly a minister, means your every word will be scrutinised in every way.
While he definitely had the ability as an economist, he did not when it came to being a politician nor a diplomat. But his resignation seems to confirm that he himself realises this.
Their government can certainly print money endlessly, but it's like issuing more stock: it doesn't just make the value of that country go up, it makes the value of its stock (money) go down proportionately.
So the fact remains that Greece won't be able to buy all the things it wants without outside cash. And who is going to give it any now? If you just print money without limit, you'll soon find that there's nothing to buy with that money due to inflation.
Maybe they can copy Argentina, but when their finance minister's exit speech is basically "haha, later suckers!" I don't have high hopes.
If their currency were to lose value, their exports would become cheaper in relation to others.
This in turn would drive up exports.
Right now the euro is trading pretty much on par with USD.
Not likely that this is beneficial for the Greek economy, as it seems very dependent on farming and raw material extraction.
Googling the actual Greek word he used brings up page after page of arguments that it meant exactly that: terrorism.