In fairness to CE, the old PR process didn’t catch Snowden either. The only thing that would have caught Snowden was one of his coworkers or supervisors reporting any suspicious or questionable behavior.
There are certain people that if they intend to divulge classified or sensitive information the clearance process will never be able to caught…especially if there are no flags, no issues developed, no self reporting, or others that saw something and didn’t report it.
In the case of the Alexis shooting and killing 12 people, he should have never received or held a clearance many years prior to the shooting. He was extremely unstable and adjudicators had enough to deny him a clearance in my opinion.