4chan

The guy who shot up all those black people at the Tops Market in Buffalo - did he really think killing random black people, would stop black people from existing?

No, he probably didn't think it would stop black people from existing. Instead, he probably thought that he had no feasible options to do so, and if he were able to do anything as an individual, then it would be carrying out a mass shooting and trying to "make an impact." These "manifesto" shooters are typically immersed in some kind of online group that promises to applaud and echo the violent behavior. Regardless, the outcome is awful.

EDIT: From an excerpt on the Buffalo shooter's manifesto:

The author also expressed support for [other] far-right mass shooters

As much as 57% of the text-based ideological sections were plagiarized

It's about being isolated from the real world, whether physically, mentally, or both, and then being sucked into a private group that idolizes bigotry/racism, violence, and other mass murderers.