Definition of extremism that I heard on a podcast recently:

"To be an extremist you have to believe that the health of your in-group is inseparable from taking some kind of hostile action against an out-group.

So It's not conditional. It's not, you know, 'we are having a dispute over a border', 'we are having a dispute over business' that we can resolve. It's a condition of your success is that somebody else has to pay for it."

Source: https://www.earwolf.com/episode/online-extremism-white-supremacy-and-the-lone-wolf-problem-with-j-m-berger/