I've personally met 2 black belts I thought were pieces of shit. I've met several purple belts that hurt people on purpose. I've seen people shove one another in BJJ gyms and get angry over rolls.
I've also seen big old fat old ladies walk away from car accidents and people drag themselves into a hospital bleeding, because they had to, or up all night taking care of someone.
I don't think there is any correlation at all between being good at drills associated with fist fighting (BJJ) and being a good person, in any way. BJJ players are the same mixed bag of good and bad people you have anywhere.
The reason people think BJJ builds positive mental qualities is because that opinion was expressed often enough by someone good at fist fighting, and some people would believe that kind of guy if he said the earth was flat.
Be a stranger in someone else's gym and pass one of their purple belts guard or threaten a sub. Sometimes he'll roll like normal and act like a human after. Sometimes all that "leave your ego at the door" bull crap gets exposed for the hypocrisy it usually is, and they start flailing and spazing, maybe even red faced and crying.