This isn't about background checks. It's not about looking at the ANA with a skeptical eye. Everyone is guessing here, including General Khan. There are a million different reasons of what could of happened but guessing Taliban infiltration after this ANA member was in for 3 years is a joke. He was also promoted so it seems his superiors thought he had value too.
another alternative option, what if this member after 3 years of serious combat, just wigged out and lost it. ANA don't get any psychological support, they are all away from home and are kept separate from everyone else. It's an amazingly tough life, a life with which we ignore because of their "warrior culture". The ADF frequently has people RTA due to the stress of a combat zone, who says this is anything different.
I'm not pushing for or against any explanation, just trying to open peoples eyes a bit from just thinking the Taliban are behind everything.