Not trying to shill or slide here but I'm a fan of Berserk (the anime) and it's well known that within the fandom fans will get the Brand as a tattoo to show their appreciation for the series, there's nothing symbolic about it other than the fact that it's an iconic image from the fictional universe because the main character gets branded. There really isn't more to this brand than that. The shooter was a fan of Berserk which is why he branded people he disliked on Twitter. There really isn't anything to these random people with brand tattoos, it's just like someone getting an american flag tattoo to show their patriotism, except these people are showing appreciation for an anime. People are devoting way too much time to this, it's a false lead. If you go and take a look at the Berserk sub-reddit you will find people posting brand tattoos almost everyday… if you want to keep digging on this be my guest but you're wasting your time.
You might be right there, to explain the brand, the antagonist in the Anime is the leader of a band of mercenaries and he sacrifices each and every one of his followers to a cannibalistic slaughter to secure his ascension to godhood. A metaphorical take away from this could be how the cabal see the people below them as nothing but sacrifices for their own personal gain, but a clause with the sacrifice is that you have to sacrifice something you care about in order for your ascension to godhood to work, in this case it always reminded me of the cremation of care at Bohemian Grove when they 'cast care to the winds' by burning an effigy so that they may continue committing evil acts to keep a grip on their power and wealth. Just my interpretation of it of course. I'm not disregarding the whole thing entirely but fans who get tattoos aren't in on the big evil is all I'm saying, they're just fans.
No problem! Keep up the good work patriot!