Regarding convicting criminals, there may or may NOT be proof of criminal activity, and accordingly, a better result may be to attempt to make whole as many customers as possible. We do NOT have enough information to make broad general statements to pursue one avenue at the exclusion of another, and we do NOT know the extent to which multiple avenues may be pursued simultaneously.
According to Mark, 600'000 BTC were stolen from his company, that is why he cannot pay to clients what they were due according to his terms of service. Whether he lied or told the truth, a crime obviously has been committed.