There's a difference between extremism and radicalism in politics, extremism usually means utilizing "extreme" methods to accomplish your goals, terrorists are extremists, radicalism is about advocating fundamental change to society. These are not synonyms, they're two different concepts.
I don't mean to drag out what looks to be a dead argument but this is a pet peeve of mine when people mix up what radical and extremist means or conflate them with eachother