This is actually the opposite of what happened. The Greek government, which was already virtually bankrupt in 2010, took a 110b EUR loan from the IMF on very harsh conditions, and Varoufakis was very vocal against it at that point already.
He was appointed Minister of Finance, he simply tried to renegotiate the terms of the deal, which the other involved parts refused.
The problem with default on a loan is that while both parties must bear some responsibility (both the giver and the taker), it will always be the loan taker that will be in a worse situation at the end (has less options, power, etc.).
(Also, I'm not saying you're one of them the way, but you happen to sound like one.)
... Negotiating debt settlements is the very first thing a financial advisor would tell you to do in case you (or an irresponsible family member) has contracted a huge debt. The fact that the lender denies your request doesn't say much about you besides your ability to negotiate. If you're dealing with a loan shark, that says more about them than it says about you.
I find normal that he tried to negotiate a better deal and that his negotiating position was worse because he was the defaulting party.
He can still (or not) honestly believe the responsibility should be more/fully with the lender. I do agree up to some point that the richer entity should bear more responsibility, but I have a feeling his discourse is often very political (trying to mobilize and get people excited) rather than propose some realistic attainable policies.
Yes, his discourse is very political, but he does not come off as a demagogue to me. I personally think that there should be a room in the public discourse for people to come up with unrealistic policies, as realistic ones always require a compromise that benefits those in already in power. It would be very hard to carve out new avenues for social progress otherwise.