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Let's suppose that the officer did stand down and this thug got away. How would you react if this lucky thug then shot his girlfriend as he had already threatened?
Would you blame his actions on the trauma of his being chased by the cop?
LEO are sworn to apprehend violent criminals. They cannot "stand down" and do their job at the same time.
The thug in this case had a gun in his hand and represented a threat to humankind whether he was running away from or toward the officer sworn to shield the general society from such vermin as the subject of this sentence.
From the little we know it doesn't seem this LEO handled it well. First of all he shows up alone, which is stupid if he's there on suspicion of a gun threat. Then he escalates what appears to be a calm conversation walking up and handcuffing a guy without any explanation of what he's doing. Neither of those had to happen. The
chase itself didn't have to happen. He could have kept the situation in view and calm until backup arrived. There was nothing apparently going on when he arrived but casual conversation (look at the body language). McMillin could have simply joined that conversation and stalled for time. They could have had him surrounded by then. But no -- he chose a different course.