The obvious answer is that an action is treasonous if it is both illegal AND damaging to one's country. Varoufakis is being singled out because his actions as Finance Minister appear to have hurt his country while the evidence he has himself provided (albeit saying he would deny saying what he was being recorded as saying!) served only to burnish his own reputation, at least to his own mind, while damaging that of his country at a crucial time.
As a Member of Parliament he enjoys immunity unless this is cancelled by his colleagues. It seems unlikely that they will turn on one of their own but Greek politics, in these troubled times, has thrown up more than one surprise. Perhaps Varoufakis will be made the 'pharmakos'- the ritual scapegoat- whose ceremonial slaying purges a collective malaise. Pity and Terror will be awakened in our breast as this great Game Theoretician whose hamartia- or tragic flaw- was that he loved his country too well to play the Stoic hand he'd been dealt, is pilloried in the agora and pushed off the steep crag of the Acropolis.
Varoufakis is a character out of Aristophanes not Aeschylus.
He has turned Greece's tragedy into a farce about one peacocking ex-Professor.
He is accused of harming his country.
His reply amounts to the assertion that he hasn't harmed himself- & that's all that matters.
Yet, he has done both.
This is his response to the treason charge. My comments are in bold.
Q: Your political sympathies lie with Syriza or at least Syriza’s end of the spectrum, but you’ve been highly critical — merciless, brutally honest — of how they’ve governed. Explain why. What has Syriza done wrong, and what could they have done better? (Related: Syriza promised it could reverse austerity but remain in the Eurozone. Did they betray voters? Did voters betray themselves with Syriza as enablers? Or is it something else?)
Syriza promised contradictory things: ending austerity, and staying in the Eurozone. Eurozone policies, particularly in the post-crisis era, are about squeezing labor.
It's hardly unusual for politicians to lie, but they normally don't tell such big lies on matters of paramount importance to voters. Admittedly, Syriza's leaders were all novices, and they appear to have genuinely believed that they could persuade Europe's leaders to change course. But as members of both right and left wing factions in Syriza recognized by the end of February, after a month of negotiating to reach an interim deal to extend the bailout, the Troika and Eurozone countries were unreceptive to Greece's "look, austerity won't work and you'll just lose more money if you keep it up" pitch. Yet [Prime Minister] Tsipras and [Finance Minister] Varoufakis refused to change course. They exhibited the Einstein definition of insanity: continuing to do the same thing, and expecting different results.
Moreover, Syriza has not lived up to its populist, anti-oligarch branding. It passed a 200 million euro humanitarian aid bill in March, yet as of May was still taking applications and has yet to disburse all the funds. Yet favored interests like the union for the state electricity utility, DEO, still has its pensions getting a subsidy of 600 million euros while other pensions are being cut. Despite massive tax evasion, Syriza has not brought any new criminal tax evasion cases. What it has done is change the members of the panels that hear those cases from judges to union reps. Similarly, during its election campaign, Syriza promised to go after oligarchs and identified an obvious target: owners of media licenses. Yet it failed to take action once it took office.
Not treason then. Just the same old clientistic rousfetia. But others were content to rob the State. You had to go and revenge porn that battered old ho bag like she was Britney Spears and it added to your rep except that's exactly what she was and so the stunt has backfired on you, paidi mou, and yours and yours alone is a now endless walk of shame.