As a parent, I may be able to offer a possible idea on the reasoning for this.
I can think of one experience that I had when my two kids were little. We were headed across a parking lot from the car to a home improvement store. I had them both by the hands while walking from the car to the front doors of the store. As we got closer, I eased my grip and they went kind of half-running to the doors. We were in the printed crosswalk, but out of nowhere a car came speeding through the parking lot towards them. I mean really speeding. I panicked and ran towards them and took their hands again and this car slammed on its brakes and honked a long angry honk. I stopped dead in front of him in the crosswalk and looked him right in the eye and my wife took the kids into the store. I wanted to kill this clown. Not really KILL, but certainly haul him out of his vehicle and teach him the finer points of the textures of scorching Floridian sun-baked asphalt and its effects on facial skin.
Of course, he drove off. Nothing ever happened except for a few four letter words and some gestures as I think he understood how angry I was. I think we were both lucky that day. I was angry at myself for not being more careful. I was angry at the kids for not sticking closer to me. I was angry that some idiot deemed that particular time to speed recklessly in the parking lot and then have the stones to honk at us for being in his way. I was even angry at being angry. At that moment, I was angry at everything and everyone.
To me, it is possible that the adult bigfoot was in a similar rage. It may or may not have been willing to kill. But it was willing to take out its anger one something or someone. The camera just would have been the final straw. It it plausible that it was angry at its child for being seen, at itself for relaxing its grip, and on the human it encountered. It was angry at everything and everyone. So it mind-spoke the threat.
That's my theory anyway. We'll never know for sure.