Looking for a term, not "radical" or "extremist" - but something that encompasses Nazis, Pedophiles, Terrorists, etc.
As an example, a psychologist might reference that "My field includes interviewing many of those who are...pathologically despised by society". (except "despised" or "reviled" may emotionally wound the subjects).
Doesn't have to be one word, but something bordering on sympathetic or at least not clearly blaming the subjects themselves. The banner under which they might group themselves.