Normalcy bias, plus shooting a suspect in the back is also, usually a career ending action.
But his career is over either way, which is why the officer now regrets not shooting the killer.
Train, train, train. It really has to be muscle memory. Do what you have to do to slow that shooter down. Practice over ands over.
If there's some small light in all this awful experience, it's that this SRO will soon be out of a job and never be employed in LE again.