An extremist is a person — usually a violent one, but not necessarily — who takes an idea really really far (from where you take the same idea).
Nobody ever mentions the second part — that extremists are extreme only relative to what you think and do — but it’s a necessary consequence of the definition of the word. “Extreme” is meaningless unless you have something to compare it to, and in the case of “extremists,” what you’re comparing them to is yourself.
Would I do this thing? Would anyone I like do this thing? What about people I don’t like, but might dislike less than I dislike people who do this thing? No? None of us? Good. Then let’s do something about those extremists.