>>29910477This.
Being a bitch means not controlling your inhibitions or thoughts and letting everyone around you suffer for it.
Being nice means keeping that shit under wraps. Even nice people have limits, which is why Fluttershy's had several outbursts over the course of the show, but the fact that she makes a conscious effort to keep her bitchiness in check and be kind to others inherently makes her a good person.
I have the ability to kill a dozen people by hitting them in a car. I don't do that because that's insane and would cause havoc and misery, but does that fact that I have the ability to do so make me a murderer?
Everyone has bitchy emotions inside them, the difference between nice people and bitchy people is keeping it under control.