extremist
 oxford
 views  updated May 18 2018ex·trem·ist  / ikˈstrēmist/ •
n. a person who holds extreme or fanatical political or religious views, esp. one who resorts to or advocates extreme action: political extremists [as  adj.] an extremist conspiracy. DERIVATIVES: ex·trem·ism / -ˌmizəm/ n.
 The Oxford Pocket Dictionary of Current English