A pretty interesting Dangenrompa-esque setting — although the idea of suicide incited by comradery never really made much sense to me.
How old/ongoing is this novel and it’s movie?
TL;DR: a small amount of people suicide from thinking they are alone, worthless, and rejected by the world around them. Dying with others sounds very good in contrast.
Humans are creatures that fear loneliness. Sometimes, even death is better than loneliness.
It comes as no surprise that suicidal people may want to at least have peers in their final moments. Especially if they are suicidal from having their friends and family ostracize them for one reason or another (social, financial, academic, or other failure, or discrimination from mental health, gender identity, etc.).
…in my case, it was social and academic failure. I’m happy now, at least.