Funny - my wife and I asked the exact same question while watching the first episode.
I think it's both. In hindsight we can see that OJ displayed many signs of sociopathy considering his lack of willingness to do anything that didn't directly benefit himself. His resistance to helping out in the civil rights movement and the reasons for it (hurting his brand, "I'm not black, I'm OJ") spoke volumes imo. As did the statements from people who worked with him regularly throughout his career stating in interviews that they couldn't figure out if there was more behind OJ than the calculated, polished cardboard cutout who was able to perfect his public persona through the 70's and 80's.
Combined with that you have the fits of rage that are common among CTE sufferers. And back in the late 60's through 70's there was no concussion protocol in football whatsoever. Football was a rougher sport, lack of medical advancements, little to no knowledge of concussions, countless carries during his 11 year NFL career, etc. There is absolutely no question that OJ Simpson's brain contains an abundance of CTE.