First off, allow me to state it is my opinion that any extremist, be it right wing or left wing, is a danger. Their inability to listen to anything besides their own self-righteous bullshit is simply appalling.
HOWEVER I do hold a special place in my bowel for right wing nutjobs simply because of their intolerance. (I really hope I'm on the ball here and we're talking about the KKK, etc, otherwise I'm about to look like a big dummy) I don't give a damn what they've been through, they can't just apply hate to everyone (i.e. racism) because ONE person of a race different from their own MAY have screwed them over in the past. That's like if I took my first bite of oranges, hated it and swore off fruit for the rest of my life. Intolerance is the highest level of human ignorance in my books.
So sure, they may be a public threat. However, unless they make any real movement, there is no reason they should be considered a threat. Unfortunately, there is no law against this sort of thing, but I guess, as long as they keep quiet, they can stay the way they are. Even if their ignorance (if by some dastardly circumstance, was weaponized 0.o) could obliterate a full-grown bull elephant in one shot, America IS a free country, so I suppose (as unpleasant as it is) they can continue their single-minded customs.