If I had to guess, I would say that Bee simply learned to reason with it.
I mean, 8-tails still seems to have a personality, and even he admits he used to be wild also, so somehow, the two of them came to some sort of understanding and now get along.
So the question becomes, are the Tailed Beasts really evil, or just perpetually pissed off. Maybe they're only violent because they hate being sealed inside people in the first place.