Final updates for now; most of the relevant information has been updated into the last article I linked, so I won't bother sourcing again.
The arsonist is 41-year old Shinji Aoba, who claims that he started the fire because the studio stole a novel from him (that old delusional psychopath chestnut). This lines up with eyewitness reports that stated he was screaming something along the lines of "you copied it". He's still awaiting official arrest while he recovers from severe burns -- I'll resist the urge to editorialize about how I think they should just slam the cuffs on him and leave him untreated.
28 of the dead succumbed to smoke inhalation / carbon monoxide poisoning; only 5 died to the fire directly. Most deaths and critical injuries happened on the upper floors as people tried to rush for the roof, as Aoba had deliberately set fire to the entrance. Unfortunately, the lack of fire code adherence seems to have contributed heavily to the number of deaths; the building had no clear emergency exits, no fire escapes, two spiral staircases that would have acted like chimneys for a gas fire (with the only other route between floors being one elevator), and apparently not even working sprinklers or other fire suppression systems. Had people managed to reach the roof in the first place, their only means of escape would have been to jump three stories to the streets below. Combined with it being a traditional animation studio with tons of paper and other flammable materials lying around, the studio would have acted as a giant furnace even if the blaze had been accidental.
Unfortunately, I can't find a list of casualties, so I don't know at this time whether any beloved directors or animators were among those killed or hospitalized. But this stands as one of Japan's worst mass killings since WW2, right up there with
the 1995 sarin gas attack (which had 20 fewer deaths, but orders of magnitude more critical injuries).