A shame because they weren't actually bankrupt - PWC has now recovered 100% of liabilities with assets to spare.
You can't treat mass psychology as an externality. Austerian policies rely in part on disciplining debtors to restore confidence in market transparency and the rule of law. If thousands of households lost their homes when their cash flow could not meet their debt payments, why shouldn't a bank go bankrupt when its cash flow cannot meet its debt payments? Lehman was just the bank that couldn't find a seat when the music stopped, and that's fair.
This is the sort of thing I mean when I say derivatives are hard to evaluate correctly. So yeah, Lehmans totally went bust because of their large amounts of trading in CDS.