ALEXANDRIA, Va. —William E. Hurwitz, the prominent doctor on trial here for drug trafficking, spent more than two days on the witness stand last week telling a jury why he had prescribed painkillers to patients who turned out to be drug dealers and addicts. But the clearest explanation of his actions — and of the problem facing patients who are in pain — came earlier in the trial. It occurred, oddly enough, during the appearance of a hostile witness, Dr. Robin Hamill-Ruth, one of the experts who was paid by the federal prosecutors to analyze Dr. Hurwitz’s prescriptions for OxyContin...