My mother passed away. House underwater. No assets. No "Estate" . No probate. Best to let go to foreclosure?

Mom passed away. Mortgage very underwater (This was a deal she made with bank about a year ago after she was in foreclosure for 6 years or so). She was three months past due after she got sick. Recently passed away. House needs major repairs. My brothers and I do not want the house. We want to walk away. Lender mentioned "cash for keys"? Nobody lives at the house - we just want to give it to the bank. Can we just tell the lender - let it go to foreclosure? She has no assets, no "estate". There will be no probate. Cash for keys requires us to clean the house out completely but honestly it will be very costly to do so. And we know that the house is not in move in condition. New owner would probably tear it down.