There are many close calls that occur where the consequences could have been dire, but there are just as many cases where the close call did not happen, and the worst possible scenario occurred. This can be due to a number of reasons, but a split second can make all the difference.
In decision making, making the wrong decision in haste can result in tragedy. However, sometimes hesitating to make the decision can cause that split difference between life and death. In law enforcement, where the suspect can pull out a weapon and open fire quite quickly, the window for reaction is quite low. This is why most cops are deemed trigger happy, as they do not want to take any risks regarding their lives.