"The couple should be locked up forever, but to what extent do lenders who surely are aware of their overextended situation bear moral (not legal) responsibility for lending to people with inadequate means?"
The answer to that depends on your religious tradition.
In the Catholic tradition, to knowingly dangle a temptation before a person who has a known weakness for that temptation is to do evil.
No lender is morally culpable for someone attempting to commit suicide. Suicide is an irrational response to heavy debt.
I don't think that's limited to Catholicism.