I doubt that the general mass of people with "extreme" political beliefs do so out of thinking that "their side" is better. It is that some positions are so fundamentally indefensible for them that there is no middle ground, no compromise possible since they are contradicting and irreconcilable. And there are some of those fundamental issues. Basic human rights are not up for discussion or compromise for a lot of people.
This comic summed it up rather well
https://pbs.twimg.com/media/DHQGsnPUQAAfMgB.jpg:large
What are you willing to compromise when it comes to the right to exist? Only some get killed?
Its of course a hyperbole, but I think thats the mechanic that leads to ever increasing factions that are completely opposed with extreme positions. You always had issues you were not willing to compromise on, but you rarely came into contact with people holding the completely opposing position. Of course there was the city whack job, but he was just that. Some guy. The point of reference was on representatives of "the other side". Politicians and so on. Which had to press (ideally of course) for a position that was the middle ground of his constituency, which was generally not the most extreme one to not scare away more moderate voters. Real conflict here came from fundamentally opposing that consensus of a large group.
With the death of political parties and movements you dont have that representation of the found consensus of the other side anylonger. Everyone can get a platform on the internet and the most extreme positions get the biggest coverage. If you hear often enough about extremist whackjobs in some group, you will sooner or later look for a group opposing those whackjobs.
https://www.aljazeera.com/news/2016/07/poll-americans-fear-e...
I think we can witness that for a while now. You dont think "your side is right", its just that the perceived alternative is absolute horror.
