The oathkeepers are not an "extremist group". I would humbly suggest that splcenter.org is wrong in this instance.
Men of authority pledging not to bow to unconstitutional orders against citizens. Actually seems noble.
I suspect they get the "radical" and "extremist" label from our culture where those who aren't on the "correct" political side are labeled a Nazi or Communist.
You did not read your own link, or you are putting all your eggs on the "McCain is a traitor" quote, which since McCain is a republican, you should love, and the Hilary rant about house to house confiscation of guns, which she has never outright advocated publicly, but has strongly suggested in speeches. That quote may be a bit paranoid, which is why it was selected from thousands of quotes to be included on that web page.
EDIT: Sorry, but there are a pretty small universe of reasons for your dislike on the page linked. I listed them. That's not mind-reading, that's just deduction.