I think that radicalism, fundamentalism, and extremism should be differentiated. They have different meanings
Extremism cannot have a good meaning because it simply means going too far. “The orange that is too hard squeezed yields a bitter juice.” it is often a label for the people whose opinions and behaviors are extremely different from yours. Extremists might not believe that they are embracing extremism.
Fundamentalism means sticking to original principles or something, and it could have a moderate as well as an extreme form. (I tend to prefer revisionism to dogmatism. Rejecting any and all revision simply because it is revision, cannot be considered logical by any means.)
Radicalism is not necessarily the same as fundamentalism because the core spirit of radicalism is thinking through something critically and thoroughly, whereas fundamentalism tends to stick to some principle, which is regarded as sacred or self-evident. ( I tend to respect radicalism. On the other hand, I am well over thirty and think that our world is often more complicated than radicals admit and everything under the sun could have both a good aspect and a bad aspect. In addition to this, the road to hell is often paved with good intentions. We always face unintended and unexpected results.)