I don't think negative interest rates will lead to default. The problem is ECB's QE, the failure of the Greek's austerity reforms, or economic collapse, which will trigger the mass default.
Why only ECB's QE and not the FED's / Japan's one? They are all alike, competing in the devaluation of currency.
At some point investors will loose confidence in governments and national bonds. Then we'll see an avalanche of selling coming in and money will flood the market causing heavy inflation.
I think we're quite close to this event. A group of speculators has already started betting against German bonds...
ya.ya.yo!