There is no cause to review how he was trained or how his actions comport with said training except to prevent it from happening again. There is no scenario which allows you to knowingly cause the death of your fellow citizen without just cause. A police officer is "a normal person" the same laws that apply to me apply to thee. If those whose job it is to enforce the law treat another officer differently it is corruption and cowardice. Cowardice is a character flaw not a justification.
“We have made men proud of most vices, but not of cowardice. Whenever we have almost succeeded in doing so, God permits a war or an earthquake or some other calamity, and at once courage becomes so obviously lovely and important even in human eyes that all our work is undone, and there is still at least one vice of which they feel genuine shame. The danger of inducing cowardice in our patients, therefore, is lest we produce real self-knowledge and self-loathing, with consequent repentance and humility.”
― C.S. Lewis, The Screwtape Letters