I'd say it's both "nature" & "nurture". I'm sure some snake breeders are way too busy to do much "handling" before snakes are sold, but if they do, it all helps. Then again, I've bred my share of snakes, & in any given group of neonates or hatchlings, some are always more calm than others. They really DO have their own personalities, but even so, nearly all can be worked with to improve how they act as pets. And that's not just BPs, it goes for all kinds of snakes. A small percentage are really chill, another small % are bonkers (lousy pets), but most of them fall somewhere in between, workable and they improve with empathy & patience.