It's not helpful to palm this off on mental illness - that does a disservice to people with such issues and doesn't address the fact that virulent ideologies exist in the real world and have proponents that are capable of planning, organizing, and recruiting.
Sure, 8ch is also full of autists who just need a hug (subject to certain terms and conditions) but if I discover that a bunch of people are making plans to kill me I'm not under any obligation to put my enemy's problems ahead of my own survival.
what if rather than being a viper pit of nefarious hatemongers seeking to brainwash the youth into committing acts of violence, imageboards tend to be popular among a subset of the population more prone to depression & more severe mental illnesses which also happens to the encompass the the kinds of loser psychopath edgelords who commit these sorts of crimes. the whole response around these events simply shows we've learned nothing from columbine and are still in essence trying to eradicate the trenchcoat mafia.