I've mentioned this before, but I'd say the worst, most extreme criminal I've ever unknowingly met was an LCS proprietor. Superficially, he always had an air of calm, happy, content detachment from the world, while, conversely, he'd suddenly make a comment with a conflicting, confusing degree of anger, contempt and sarcasm.
However, what none of us actually knew at the time was this, and it does explain his somewhat polarised behaviour...
https://www.manchestereveningnews.co.uk/news/greater-manchester-news/article-ronald-castrees-murder-lesley-7831883
Quite appalling. Even to this day, thinking about the interaction I had feels quite weird.