fanatic

Fanatical.

But Faith, fanatic Faith, once wedded fast / To some dear falsehood, hugs it to the last.

Showing evidence of possession by a god or demon; frenzied, overzealous.

A person who is zealously enthusiastic for some cause.

The reclusive tycoon calling himself the Illusive Man is a human nationalist focused on advancing human interests, whatever the cost to non-humans. The Citadel Council regards him as a fanatic posing a serious threat to galactic security.

A zealot can't change his mind. A fanatic can't change his mind and won't change the subject. —Winston Churchill (attributed)