How did they get it wrong ?
They shot someone who wasn't actually a threat. In an ideal world, the police wouldn't do that.
Of course, we don't live in that ideal world and we never will. Police officers faced with life-or-death decisions to make in a split second can't get up to that standard. In this case, it seems that the police involved acted in a reasonable fashion that falls within the standard of lawful killing or cleared of wrongdoing but that doesn't mean that their actions were as excellent as they could be. The aim should be to learn and do better next time.