People don't just 'snap' out of the blue. Even if they have never shown violent behavior before, this doesn't mean too much. People often put their best foot forward in public and hide their darker side. And stressful events can be the trigger which unleashes what has been brewing in them.
Patsy Ramsey for example has always been presented as a very indulgent and patient parent of whom her supporters could not imagine to have snapped just because JB had wet the bed just one more time.
But there is an interesting passage in Thomas' book (p. 326/327) where Patsy becomes aggressive during the interview with Tom Haney (bold type mine):
Haney said they were not ready to show her evidence and challenged her further. "Pal, you don't want to go there, " she warned, adding that she was a good Christian woman who did not lie. She pushed back against the couch and exhaled in disgust. "Criminy", she explained.
Haney continued to be inhospitable and probed about whether the death could have been an accident resulting from bed-wetting. Patsy held up a hand, like a stop sign. "You're going down the wrong path, buddy!"
Later she said, "If John Ramsey were involved, honey, we wouldn't be sitting here. I'd have knocked his block off. Read my lips! This was not done by a family member. Didn't happen. Period. End of statement."
Still Haney came on, polite but insistent, inquiring about any family secrets, and she tired of him. "Cut to the chase", she barked.
"Oh, no," Haney responded smoothly, "That would spoil the ride."
"Then spoil my ride", Patsy said, riveting him. She didn't give an inch.
It was a spellbinding exchange. Tom Haney, with his no-nonsense style and three days in which to ask his questions, had found something I felt had to be there somewhere not too far below that polished beauty queen surface. Patsy Ramsey had, for a few minutes, lifted her mask. Beneath it, I saw cold rage.
Too bad the Ramseys have never been psychologically evaluated. What do we really know about Patsy's childhood and possible detrimental influences which may have shaped her character? Another dark area.
Burke btw when interviewed said that JonBnet's bedwetting was a big problem. And to someone like Patsy who wanted a perfect little doll as a child, JonBenet's silent protest through bedwetting was probably a continuing insult to her mother's narcissistic personality.