Even though he was finance minister he was/wouldn't have beeen not authorized performing some of the actions done (e.g. "A secret cell at the Greek finance ministry hacked into the government computers") or planned. Even the prime minister has not the authority for such decicsions. As in other democracies the parliament has to vote. Calling it treason depends on the point of view. We'll never know wether it would have been for the country's advantage. I highly doubt it. However, Varoufakis actions were illegal and dictatorial. They were an act against democracy and therefore I guess, yes, they were treasonous.