This sounds very believable. These predators are not like us. They lack empathy and compassion and yes, they may belong to a club and want to "keep up" with each other. I would say it is safe to assume that they would take photos of what they have done. This stuff may be posted but due to the onion router [TOR]/use of the dark web, finding them is extremely difficult. I would assume LE would have to embed an FBI agent/someone...to slowly get into the "group." It would be painstakingly slow and very challenging. All IMO.
(Sorry for answering for the other poster).
I was thinking about teenage groups who view "mutilation". They are more common than you'd think and VERY concerning given their age and the fact that their brains has not fully developed. When I asked myself, for how long the killing "club" or the "community" might exist, I thought of two extreme forms of control, homicide and suicide... should have formed at the same time for the two, just logically, because, human behavior. So, one can have a guess. Definitely with AOL I think, so, beginning of 2000es?
There might be different ways to find them out. Give the job to someone logical. But I am thinking, that if, just if, BG belongs to one of such groups, he does not need to be too smart nor needs to have special knowledge. He might get all necessary ideas from the club, including helping with the planning. I am thinking about it more and more.
Has he killed before? I don't know, but the planning shows some knowledge. Either, a genius, or, got some advises. Probably. I bet, some animals, and not in a nice way.
Has he killed after? SKs escalate in their behavior, because, as I suspect, there is a strong OCD component in it. But a collector's behavior, while being similar to OCD, in fact, a form of OCD, is different - people can wait for a very long time to find what they want, while constantly searching. So, one thing, he really got burned with the photo, video and recording, another one, he might be in the process of looking.
Think of this. Collectors have private clubs, Facebooks, IGs, any media. But something "cheaper", or just "parts" of their collection, they might post online for everyone to see. What do you think SK could post openly without being afraid of getting caught? I have two ideas. Places. And tools.
Think of this cretin, this moron, who found a group to kill for him in AK. None of the participants was a genius, and yet, a girl was killed. And he looked like a nice guy!