This link answers my question - thanks.

Far left is the opposite of far right, for which we can all point to many of them that have been banned from twitter.

I'd define the far right in terms of distance from center, personally.

So, I'll refine my question with an example from the list: who is the Anders Breivik of the left?

well helping people is the opposite of killing them so maybe a meals on wheels driver or a public defender?