"...what prompted him to act at that particular time?"
I've wondered about this, too. Since all behavior is multiply determined, then, if it was BR, I think a larger pattern rather than any one incident holds the clues. And a sustained, “dots connected” narrative of BR’s life is something missing from the case. What follows is only a bare bones version, based on key events we know and statements BR has made. It's not really adequate, but maybe it's a start to understanding more about the timing, the anger, and the overkill. It all has to do with being 6 ½ years old.
BR's birthday acquired associations with death and Christmas. His half-sister Beth died on 1/8/92, two weeks after Christmas, casting a pall of grief on his 5th birthday barely three weeks later. The following summer PR was diagnosed with end stage cancer and began commuting to Bethesda for experimental treatments. Nedra and the housekeeper became the children's caretakers. BR was 6 ½. PR was so weak and immunodeficient that she isolated to JAR's room. By Christmastime 1993, as BR approached his 7th birthday, it wasn't clear whether PR would survive.
She did survive, and in the latter part of 1994 was recovered enough to enter JBR in her first pageant. Around Thanksgiving in 1995, PR was declared to be in full remission. By then all the pageant efforts - the contests, costumes, lessons, photoshoots, travel, spending - were in full swing. Too full. Scarcely a year later, several family friends felt that the whole "mega JonBenet thing" had gotten completely out of hand and planned an intervention with PR. They might have noted that such an excess creates a corresponding deficit, that BR was neglected. At 8 and 9 he had school, friends, scouting; but nothing replaces a mother’s time and attention or makes you feel okay if you know she's spending a ton more money on your sister than she is on you.
What happened to BR during those two years? When PR became ill, BR was the apple of her eye. Suddenly, he had almost no contact with her for months. The pain was so overwhelming that it had to be bound in anger, anger that drove him to smear the bathroom wall with feces. He had felt worried and afraid; he had missed his mom; he had been brave; and he looked for some acknowledgment of that, of what he had come through. As PR regained her health, BR also expected, as children that age do, that things would simply go back to being the way they were. Not only did neither of these things happen, in 1994 JBR - his sunny, beautiful, sometimes bratty little sister who teased him, smashed his legos, interrupted his playtime with friends, and acted bossy - JBR replaced him as the golden child.
At Christmas 1996, JBR was the center of her mother’s world and destined to stay there for the next 13 years until she was crowned Miss America. At 6 ½ her life looked great. What a contrast to BR’s life at 6 ½. Through no fault of his own, he lost his mother for months and life as he had known it. His family seemed not to notice that loss. The difference in his and JBR’s fortunes must have felt deeply unfair to him. The family had no place for his grief or his outrage. Studies show that fairness is a universal value and anger a universal response to injustice. Children in particular feel injustice keenly.
In December 1996, the parents attended or hosted party after party and gala events. In between these, on December 6th, JBR rode in the Boulder parade on her own float, “Little Miss Colorado,” made by PR’s father. On December 17th, JBR also won the “Colorado’s Little Miss Christmas Pageant” and won a prize at a separate event near Denver. On December 20th, JBR also spent an entire school day performing her latest routine for every single class in her school - which was BR's school, too. On December 22nd, she also participated in a pageant at a nearby mall. The intense focus on JBR’s outward appearance matched the indifference to BR’s inner world. Maybe by Christmas BR had just had enough.
Young children process logically, symbolically, unconsciously, in make-believe, and magical thinking all at the same time. Surely, in BR’s mind JBR didn’t deserve so much attention and applause. Maybe his mom would pay more attention to him if she weren’t so busy with all the pageant stuff. Maybe it was too painful to think that his mom would stay wrapped up in JBR forever; that there was no way back to being the apple of her eye. The summer he was 6 ½, his happy life ended abruptly in Charlevoix when his mother’s cancer was discovered. Maybe it made a kind of sense that if his father’s favorite, Beth, died suddenly around Christmastime, right when everyone was happy, then if his mother’s favorite, JBR, also died suddenly at Christmastime, it would make things even, in a way. Maybe if something happened to JBR, the family and PR’s frantic pace would be calmer for a while, the way they were after Beth died. Maybe if they could go to Charlevoix without JBR, things would be okay. Maybe if JBR’s life ended at 6 ½, that would make things fair. It would be a kind of reset that made it possible to get on with his life - his real and rightful life in his mothers' doting regard, the one that stopped in Charlevoix in 1993, life as he’d known it before cancer and the pageants and mega JonBenet.
Admittedly, I can’t know the details or exactly how the dots connect, but, from my studies and work, I believe that, if BR killed JBR, the motivation and timing emerged from his mental and emotional associations, conscious and unconscious, of death, Christmas, mother, profound loss, grief, anger, and Charlevoix. However things unfolded on the night of 12/25/96, I believe that immediate events themselves were not the trigger but what those events represented to BR.