Short answers, pulled from the story.
Religious fanaticism, also called religious extremism, refers to uncritical zeal or obsessive enthusiasm tied to devotion to a religion. Psychiatry uses the term hyperreligiosity for the same phenomenon. Scholar Lloyd Steffen identifies three core features: deep spiritual need, the movement's attractiveness to sincere seekers, and its presentation as a live moral option offering meaning, power, and belonging.
In Christian antiquity, the term was applied to denigrate non-Christian religions rather than to describe dangerous behavior within Christianity itself. The Age of Enlightenment later gave the word its current, broader meaning as a designation for obsessive or destructive religious zeal in any tradition.
J. Harold Ellens argues that Christian fanatic rule began with the Roman Emperor Constantine I. When Christianity came to power under Constantine's empire, Ellens writes, it proceeded to repress non-Christians and Christians who did not conform to official Orthodox ideology, policy, and practice.
The Kharijites emerged in the 7th century CE during the First Fitna, the first Islamic Civil War. They broke from both Sunni and Shia Muslims and adopted a radical approach to takfir, declaring other Muslims to be infidels or false believers worthy of death for apostasy. They are considered the earliest example of Islamic extremism by scholars.
Sayyid Qutb was an Egyptian Islamist ideologue and prominent figure in the Muslim Brotherhood who promoted pan-Islamist ideology in the 1960s. After he was executed by the Egyptian government under Gamal Abdel Nasser, his ideas were carried forward by Ayman al-Zawahiri through Egyptian Islamic Jihad. His books were later cited frequently by Osama bin Laden and Anwar al-Awlaki, and his ideology influenced al-Qaeda and ISIL/ISIS/IS/Daesh.
In Arabic, jihad means struggle. Lloyd Steffen identifies three traditional forms: living out Islamic values daily, countering arguments against Islam through debate, and self-defense when physically attacked for belief. Steffen reads the Quran's military uses of the term as always defensive and subject to restraint. Osama bin Laden's fatwa in 1998 reshaped the term in public discourse to describe offensive violence, which scholars note contradicts its traditional meaning.