Mortgages they bought after receiving assurances of the gilt-edged nature of these securities. These assurances proved to be wrong/fraudulent. The folks who did that also bear some of the blame.The lenders left holding worthless mortgage were (and are) primarily pension funds and retirees.
Be that as it may, the borrower owes the bank exactly what was in the contract, subject to the enforcement measures in force at that locale. IMO that's all they owe--but they do owe it. This wasn't a loan from a family member or a charity, it was a loan from a business that was making a business decision.