Vadim Perelman was a young hedge fund hot shot not so long ago, overseeing an $850 million portfolio at the age of 31, hopping on corporate boards, and issuing lengthy slide presentations about his favorite stock picks. But then Perelman placed an enormous bet on a single company, Walter Investment Management. This week Walter Investment Management, a Tampa, Fla., mortgage servicer and nonbank lender, said it lost $529 million last year and recently hired restructuring experts to figure out a way to reduce the company’s crippling debt load. The news was bad for the mortgage servicer’s shareholders, but it was particularly...