Homebuyer walking away when house price down, net equity zero ..
... in most cases will be in for a surprise.
When the bank repossesses, then advertises the property for sale, if the value has gone down even farther, and they sell it for less than is owing ...
... they'll usually chase to buyer for the difference.
Suppose the mortgage was for $140,000. and they end up with $120,000. net on the sale, after the costs involved ...
... they'll be telling the former owner that s/he owes them $20,000.