Edit: Here’s a link to what is most likely the real manifesto: https://www.kenklippenstein.com/p/luigis-manifesto
Ken Klippenstein is a very reliable journalist and this version of the manifesto contains the snippets that have been released by law enforcement. Also, considering the thing was hand-written, that very long version involving his mom is dubious. (And there’s not any good evidence that his mom is in anything besides decent/good health)
Pain makes people desperate. Desperate people are not known for being particularly calm about things, and it is entirely unreasonable to expect calm and decorum from someone in constant pain (that can be improved if not eliminated by certain treatments that are unfortunately usually quite expensive).