I would agree with your premise 20 years ago. Or maybe 10. Maybe.No I don’t, but I think if there is ideology in the manifesto that others relate to then it might influence them to act.
Hypothetical - let’s say the manifesto says group A tries to limit, persecute, infringe on group b and group b needs to rise up against group a. Then other members of group b that may feel they’ve been treated poorly by group a may then feel they need to act against group a.
Clearly anyone who really thinks and feels that violence towards others solves anything or is the answer needs help. They aren’t thinking clearly and probably have untreated mental health issues.
But social media, the internet, etc has really made most of this moot. There's a hate group for almost everyone. There's a safe space for everyone who holds a grudge or dislikes one group or another. There's a "gang" who tells you/or your child that they will be a new family and that they love you/them. They hold the same "values". White Supremacists. Pedophiles. Antisemetics. Religious Cults. Activists of every sort.
If manifestos, detailed books about serial killers, etc had that much influence in this country, we'd have a lot more than 100 (or less) people killed in mass shootings each year. I'd blame video games and music first.....and I don't think that has much of an impact, either...but some folks do. All imo