The Brand is from an anime/manga called Berserk which the shooter appeared to be a fan of as he posted clips of it to his twitter, as far as I'm aware those posts are still there unless his twitter has been taken down already. The protagonist of the anime has the brand and its a popular icon among the fandom (a lot of berserk fans get the brand as a tattoo), whether it has any other connections to freemasonry, I'm uncertain, but it's not likely.