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)
Apologies in advance for severe treatbrain
Steven Erickson’'s Malazan series had a big bad introduced in the third book named the Crippled God who was the personification of this, a mind subjected to terrible pain with no rehabilitation in the aftermath of its trauma. This god quickly establishes himself as the biggest player in the game, with torturous power that contorts people’s bodies, and so pain is presented as something fundamental, something antithetical to meaning that is more powerful to living creatures than the spheres of any other God.
To counteract the treatbrain I’ll also bring up the fact that solitary animals do not tend to cry out in pain like social animals do. They do experience pain, even though some people will try to classify it as something lesser with words like “nocicpetion,” but the important distinction is that they aren’t usually sent into fits of screaming and paralysis like social animals. This tells me that, for social animals, pain is something that MUST be answered, even to the point of wanting revenge when the pain is gone. The CEO (I spit on him let’s be clear) would be alive today if Luigi’s pain had been addressed properly. But if this CEO had pushed such a policy, my guess is that he wouldn’t have been made CEO in the first place.