I think this may vary by state, I am sure others will have a more detailed response. The basic factor would apply. If the retreating BG has commited a felony (killed someone) then you may shoot them. Think about the recent Lakewood officer's killed and the BG running away. I OC or CCer was in the coffee shop and witnessed the incident and does not have a shot until the BG is running away, this would be a justified shot to eliminate the future threat of this BG.