Notorious (as a subgroup of famous) would have been a better word choice. They got the point across reasonably well otherwise, however.

I think by fantastic the 4channer meant "newsworthy", or "that really affects people's lives". The chances of someone with that kind of background doing something fantastic in a good sense is really small

In this context I read fantastic in a morally indifferent sense, as in it set him apart from others and allowed him to leave an impression on the world, albeit a hugely harmful one.

Fantastic could mean unusual or extraordinary.

Problematic is that some people try to frame it as if his problems were the cause and reason for his actions. While obviously the point where people turn into mass shooters is when they decide to hate and blame (a specific group of) other people for it.

There is far from enough help for people who are struggling, but to prevent mass shootings the media probably shouldn't talk about them this much and we need to look at people much closer who turn their hatred outwards.

Fantastic has the word fantasy as its root, but the meaning has shifted and that usage has fallen out of favor a bit. The same happened to terrific, which is an even greater oddity, terror being the root. The act was both fantastic and terrific but not in the most common contemporary usage of those words.