After officer charged, Amtrak shooting detailed: Cop 'assumed a crouched firing stance'

Sad case of lack of judgement by the Police Officer, with the victim that far away and running away. Why was he engaged in the first place, is my question? I have to also mention that just weeks before there was a firefight on a Chicago METRA train that just departed Union Station and so in this context the officer was probably nervous to boot.