Perp confirmed to be a train otaku (there are different kinds of otaku) mad about the KyoAni building attracting a pilgrimage of anime otaku and ruining the scenery near a train spotting location. Plus, Hibike had a baritone sax solo that people abbreviated as "barisaku". Coincidentally, "barisaku" was the abbreviation he personally used for another train spotting related term, バリバリ観光してサクッと撮りましょう.

the guy that burn kyoani is a train otaku (the train fanatic basically) who hate the fact that hibike (anime by kyoani) bring a lot of otaku to train spotting location
also that the sax use same term as train spotting
tldr; train otaku dont like that the anime bring normal otaku to his spot so he burn the whole place down