@MxSadie barring those with vested self-serving interest in this, I think some of it is due to the fact that they may have met someone who claimed to be right-wing because of specific topics/ideas, and got better when a wider context was shown or explained to them.
But this misses (among other things) the critical point that online it's never just one dude venting, but a whole army of people and bots, who can "support" each other in being awful, and who are backed by a lot of money.
Moreover, this doesn't excuse from listening to the people who have witnessed, or have been through, these dynamics already. And this step (listening and accepting other people experience) is crucial, and something I would expect to not need explaining.