I find the psychological problems he lists to be odd.

Has there ever been a study of civilian self-defense shooters having a great-than-normal occurrence of this?

I have never shot a man, but I know a couple of people who killed men trying to rob them at knifepoint, and neither of them had issues. Sleep soundly, glad they were able to defend themselves, etc. And I know Cooper wrote extensively that the main emotional response he heard from people involved in self defense shootings was pride, not nightmares or psychological issues.

Where does this come from? Is it a fiction perpetuated by the rabbit press, or is there some actual truth to it?