DERRY – A dog owner shot and killed a pitbull Sunday afternoon when it attacked his dog in the owner's back yard, police said yesterday. Police said the incident took place about 5:30 p.m. at 35 South Ave. The resident told police he had been having problems with the pitbull, which lives in the neighborhood, said Sgt. Eric Kester. Kester said an investigation is continuing, and the names of the shooter and pitbull owner were unavailable last night. Kester said the pitbull had bitten the South Avenue resident's dog several times, and a warning shot did not deter the attack....