Yeah, the cognitive decline could be due to excessive drug use (ketamene). Plus the side effects of hormone imbalances on a person with malignant narcissism.
I have a theory that the anger issues could lead to cognitive decline. Anger and rage has a tendency to kick off the fight or flight response. With that comes survival mode, lizard brain thinking, and irrational decision making. There's a lot of viral videos of people in this situation where the anger is causing them to make bad decisions. The experience might even be traumatic for them, kicking off their mental defense mechanisms.
Afterward, they post-hoc try to justify what happened in those moments to hold onto the idea that they are still a good person. Occasionally they realize they have a problem that needs to be fixed, but many times they will dig into their irrational positions with new emotional attachments to it. It can take a lot of therapy to get past it, which still has a lot of stigma with testosterone-types, so they don't and the irrational "logic" builds up over time. Therefore, cognitive decline.
Are there more substantiated studies on the effects of anger, especially there we see this pattern? I think I am seeing it over and over, but I can't be sure of the prevalence of the behavior since there is a bias in the media I see.
According to Dr. Mike: steroids/testosterone, depending on what type and how much, can cause: anxiety, intrusive violent thoughts, a "marked, proximate reduction in IQ," and "an inability to perceive a broad spectrum of positive human emotion."
He (Dr. Mike) openly uses "gear" and has a PhD in sports physiology, so I consider his opinion valid. You can hear more about his thoughts on the subject (while literally feeling those effects from the steroids he uses) here: