kappas???

I understood it as that's what the children called the rapist (or the man they didn't like, who was revealed to be a rapist)
like a nickname the children gave

I had to read it a couple times to come to my conclusion. Kappa is a man, and A and B are estranged childhood friends as a result of Bs trauma. B was the one in the video, and the tapes had dated spanning multiple years so I think he's a victim of continued (pedophilic) sexual abuse