Bush war in Afghanistan in 2001

You and I know that at a formal level the Bush war was over after it was waged for some 11 years.

If this is true, then how is this possible:

http://www.aljazeera.com/news/2017/06/troops-killed-insider-attack-nangarhar-170610143131831.html

Yes, now I remember.

It was over, but, the US remained to train the Afghan police and military. 

Well to do that the US officers had to have some human connection with the men they were training - friendship, alliance, respect, loyalty. 

So, how come this US officer failed to secure the friendship, alliance, respect, loyalty and regard of this trainee?

What happened for this Afghan trainee to so hate this US person as to shoot him. He certain had many other possible US targets and he elected not to kill any other but this US officer?