I have several thoughts on this...

First, I suspect that Watson (while not technically a sociopath, according to prison psychiatrists) had incidents in his childhood where he pushed himself to do harm, just to try it. We know that by the time he was in college, he was dealing drugs and was promiscuous and careless and even unlawful (the typewriter incident).

Second, I do believe that over the course of a year's time he did so many different drugs, notably the combination of LSD-belladonna-speed. Each has different impacts on the brain and the psyche, and together they could have formed a rather horrendous cocktail that unleashed something vicious in his personality, something dangerous and unpredictable and almost demonic. The fact that Watson can recall everything he did in 1969 including some of his strange behavior during the belladonna incident makes me wonder: could his personality be so drastically altered as a result, and yet still recall it all? Hmmm...

Lastly, I think there is something inherently dishonest and unscrupulous about Watson. It wasn't just that he was dealing drugs, but burning people (maybe robbing them too) in the process. It wasn't just that he killed on Charlie's orders, but did it so viciously. It isn't that suddenly he discovers Jesus, but that he becomes a prison minister - and thereby exploits what should be his life behind bars by using it for financial and other gains.

I think he's a high-functioning Type B personality - I just don't know which Type he may be. If not a sociopath, what is Charles Watson? Take a look at this list and tell me what you think: https://www.mayoclinic.org/diseases-conditions/personality-disorders/symptoms-causes/syc-20354463