In several comments throughout here I have described from experience with accuracy how these kinds of phantom employees exist and the nature of the organizations that provide the conditions for them to exist. I get that portraying government as anything but pure unadulterated competency at every level is not ideologically acceptable to many here but unfortunately that is the reality. Nevertheless, even in highly dysfunctional corners physical threats of violence are not the norm.
Do you seriously think that this flew under the radar for 15yr because he made threats? No, it was because the organization was dysfunctional in many ways and his behavior was not far enough out of line to cause problems back when he did it.
I hope you one day inherit management responsibilities in the kind of organization that has this kind of dysfunction sprinkled throughout it.