Other forms: extremists
Nowadays, we tend to associate the word extremist with "terrorist," and use the two words almost interchangeably to define someone who holds radical political or religious views.
An extremist can hold views on any subject: if you believe all little brothers should be banned from their siblings' rooms, or that anyone who roots for a different baseball team should be quarantined, then you, my friend, are an extremist. What tends to unite all extremists is their willingness to act, often violently, to realize their beliefs. Little brothers, watch out.