We have a bitey adult. Some are just wired that way I think. There's a youtube video of an entire clutch of newly hatched babies all biting each other. We have greatly curbed this behavior by giving a warning before we pick her up by rubbing her back gently. Then if she doesn't hide her head or have it neutral upon lifting out, I cup my hand and put it over her head and then put my other hand over her body to let her know I'm bigger than she is. It's all very gentle. It seems to work well for her, but I can never fully trust her and I expect her to bite still at times.
The other day, against my advice, my daughter took "bitey" out and had her friend holding her. Which was all going well until a couple of other kids burst into the room and crowded around her, and she bit the friend and made her bleed a bit. The mom was cool about it but I told my daughter that no one but family holds that snake in the future, nor does she come out when guests are over.
Now my other BP is a totally docile snake who never has bitten or even threatened to bite anyone. I do think some of the personality is just wired in.
Edited to add the video showing the bitey babies and tips that the keeper shows to help with this behavior.