Now that has changed, and everyone, however disorganized and deprived of resources, can publish their every thought. People who think they’ve been “gang stalked” and people who think that the government is behind contrails all have a megaphone to the world. As a “bonus” they also are able to just type their delusions into Google and find sites to confirm and expand them. The same network effect that connects hobbyists and enthusiasts of rare things around the world does the same for people with delusional frameworks.
It also does the same for Holocaust deniers, and general shitheads.
It's hard to phrase these things. I wouldn't say people were less confident in the delusional beliefs. Rather, an isolated delusional person has less general confidence - the world is "against them" and their relative weakness relative to the world is fairly evident.
Overall, success at doing horrible things requires some of the same mix that success at doing constructive things needs; confidence, support, plans and ideas. The process you describe and we all can see helps in all those things.
That said, I think, historically, a substantially part of the reason mentally people usually are harmless is because they can see their grasp on reality is shaky. Others have generally not shared their delusions and so no one validates their desires to lash out at those who they imagine have harmed them.
The Internet seems to have changed that - communities of people who share pathological illusions seem to have come together and produced a still small number of horrendous crimes - the "incel movement" being one very visible example.
These folks might well in dangerous (to others) gray area between debilitating insanity and approximate sanity. Pathological enough to embrace problematic ideas and functional enough act on them.