A San Quentin prison convict was shot and killed by a guard on a gun rail for refusing to halt his stabbing attack on another prisoner, officials said. The guard in an East Block exercise yard saw the assailant pull a homemade knife from a pocket and stab another inmate in the upper midsection, Sgt. Dave Langerman said. The officer ordered the convict to “freeze” but the assailant ignored the order and stabbed the victim in the neck, Langerman said. When the assailant grabbed his victim from behind “almost as a shield” and appeared to be about to stab him again, the officer shot the attacking convict in the abdomen, Langerman said. The assailant died at an outside hospital. His stabbing victim was reported recovering satisfactorily at the prison hospital. Neither man was identified.