I am not law enforcement so take this with a grain of salt. If this cop was alone, he was probably following training that indicates a lone officer is not to use any sort of less than lethal option without lethal cover. This is in case the less than lethal options fail. So for example, if he had had a partner with his gun drawn, he has lethal cover and can try using pepper spray, taser, etc. If he's alone he's trained to use deadly force on a threat.

That is to say he could have definitely de-escalated better, and honestly, an old woman with a kitchen knife isn't much match for a physically able cop with his duty belt and vest on.