Private equity and hedge fund firms have bought more than 100,000 troubled mortgages at a discount from banks and federal housing agencies, emerging as aggressive liquidators for the remains of the mortgage crisis. But the firms are now drawing fire from housing advocates and lawyers for borrowers who contend they are too quick to push homes into foreclosure and are unaccommodating when it comes to negotiating loan modifications with borrowers. Federal and state lawmakers are taking up the issue, questioning why federal agencies are selling loans at a discount of as much as 30 percent to such firms.