San Francisco prosecutors have charged a 51-year-old Tenderloin resident with murder for a fatal stabbing inside a residential hotel last week, allegedly during a dispute over drugs.Alex Herrera was scheduled to make an initial appearance in San Francisco Superior Court this morning on charges of murder and robbery for the killing of 37-year-old John Montgomery Thursday afternoon. The stabbing happened about 12:30 p.m. at the Vincent Hotel at 459 Turk St.Herrera and Montgomery lived in separate rooms at the hotel, which provides housing for the homeless and is run by the Tenderloin Housing Clinic.Prosecutor George Butterworth said Herrera tried to rob Montgomery of methadone. When the attempt was unsuccessful, he stabbed Montgomery, Butterworth alleged. Shortly after the stabbing, hotel staff detained Herrera about a block and a half from the hotel. Police then arrived and arrested him.Butterworth said Herrera has a prior conviction for robbery in San Francisco.