2012 - Kang’s security clearance was revoked because of concerns about the death threats and Islamic State comments.
216 - A confidential FBI informant reported that Kang was talking about radical Islam more frequently. 
2017 -  Kang told an FBI informant that he had been researching online videos about the most effective and painful torture techniques....Kang was then arrested by an FBI SWAT team without incident.

This certainly wasn't any rush to judgement!

This demands an independent investigation. Why was this obvious religious nutcase, I mean fervent believer, allowed to remain in the army for so long?

How many times in the past have those who have gone violently berserk because of some ideology or religion, been known, and questioned by the FBI or other security authorities?
I suspect there's more to this story than what we presently know.