Standdown allowed shooter to pick off students with no interference.

This article puts it nicely.

Eyewitness accounts describe police hiding behind trees and failing to pursue the killer, while ordering the school to be placed on lockdown so nobody could escape the carnage as the killer picked off his targets with seemingly little interruption from the police. [+] This means the the police department are possibly involved in this killing spress. They cooperated with the standown.