Varoufakis didn't have the final say on if Greece left the Euro. It has been quite public that he disagrees with a lot about what unfolded, including the decision to remain given the outcome of negotiations.
The idea that people were clueless about what would happen to Greece if Greece left the Euro is also ridiculous. Plenty of economists, in addition to him, opined on this matter. No one has argued that Greece would be instantly more well-off, but that they would be better off over the long run given their set of problems.
There have been efforts to try to use Varoufakis as a scapegoat for what happened, even though the crisis existed before he came into power, and Greece's situation is too complex for one person to take the blame.
Tsipras received support from the people during the 2015 referendum on the bailout. People overwhelmingly rejected the bailout. Tsipras balked and didn't follow through even though he could argue a public mandate.
The result of such an approach was exiting the EU. To say that Varoufakis didn't called the shots on that is naive. Pretty much everyone understood that if we were to push Germans too much we might end out of the Euro. When Tsipras eventually realized that Junker and the EU had made contingency plans for such an occasion he came to his senses realizing that the bluff has been called. All these are documented in various sources.
The crisis most certainly existed before Varoufakis came into position and no one is blaming him for that. What we blame him for is that he never had a plan and dragged the negotiations for far too long which eventually ended-up with capital controls because we run out of money.
Come on...
In almost every government, its an executive/cabinet level position which means all his power derives from the President or Prime Minister. He doesn't take independent action, as he has no independent power.
And by the way, those obligations you speak about were not created by corrupted governments. They were created for sustaining an unviable model that gives pensions to people at their mid fifties and maintains a soviet-era type of economy where the state controls pretty much everything.