Just looked this up to see if anything new had happened. Apparently the dude that did it suffered burns over 90% of his body and has somehow survived it. In fact his case is probably going to be studied pretty extensively because burns over that much of the body are 99% fatal... and the skin grafts used for him were sourced solely from what little skin of his own he had left, and synthetic grafts.
Apparently despite being mentally ill he will still be arrested once he is deemed capable of living without medical assistance.
https://www.japantimes.co.jp/news/2019/11/27/national/crime-legal/suspected-kyoto-animation-arsonist-undergoes-unprecedented-skin-grafts/
If the dude is deemed mentally competent to stand trial no doubt he'll get the death penalty, which really makes so much effort to save him seem moot, were it not for his contribution to science.