Why fire at all in the given setting? Instead of "Wait for backup, clear out the area"? Retreat out of the immediate danger area, take cover, take time to asses

Why fire at all in the given setting? Instead of "Wait for backup, clear out the area"? Retreat out of the immediate danger area, take cover, take time to assess the situation. The general way things go wrong seems to be "police arrives at scene, pulls gun, pulls trigger until clip is empty." [1] - in this case multiple officers, even after the assumed hostile was on the ground and not moving. I'm fine with a deliberate assessment and a deliberate kill shot - it's warranted some times and in that case shoot to kill and make it count. It's just that I don't see the assessment. [1] the kid on the playground in cleveland, the guy in the supermarket aisle, ...