Maybe he got AIDS and then saw his spree as a way to:
1. Avoid a long, expensive, embarrassing degenerative process
2. Make things right with Allah after his debauchery
3. Strike a blow against the great Satan, America
4. Get back at the undignified, secular drama-seekers that had infected him
5. Obay orders from higher-ups?
???
His father wanted the US State Department to help make him President of Afghanistan. This would not help with State Department support. It would however help with convincing the Taliban that his family was genuine Islam. They all knew what was planned. My guess is hit and run to get video evidence for Taliban support. But something went wrong. Got stuck. Ran with ISIS story, which the white house is trying to scrub.