With the internet, they can build self-reinforcing “communities” on the basis of hate, intolerance, or really anything.As you say, such "communities" existed before the Internet. It's just that now, they can better organize to affect the real world. I think Forrest Gump's observation applies here. Even 8chan had some sort of ideology justifying what they said and did. Even the worst scoundrels of history used words like "love" and justice." (And yes, Godwin's Law comes into play if we go down that rabbit hole! So let's not.) If we are to gauge how toxic a group is, I think we need to go to first principles and measure how driven by viral outrage an online group is.
It's natural for people to come together because they are outraged. However, when continuing the cycle of outrage, driving more attention, driving group membership, driving more outrage continues for its own sake, there is a problem. It shouldn't be outrage and attention driving a group. It should be higher principles.
Unfortunately, outrage is a fundamental fuel of virality in 2019. Today, we need to be skeptical of actors who lean too heavily on the cycle of outrage, whether they're openly reprehensible, or if they merely dress their movement in high language.
https://www.youtube.com/watch?v=rE3j_RHkqJc
In 2019, the world has long known to be skeptical of religions gone wrong, which we call cults. We need to develop the same kind of skepticism to groups actually ruled by outrage, not principle.