Def. - When people hold harmful or very far from what is considered normal religious or political beliefs.
Radicalisation is when someone becomes extremist.
Why?
Influenced by others such as friends and family, online radicalisation, people believe they are doing the work of God, believe they will be rewarded in the afterlife or interpret their holy books in a way that makes them think they are supposed to be extremist.
Examples:
KKK - white supremacy, kill people who disagree. Taliban - stop girls going to school. Mao - most deaths in human history, led communism in China. Hitler - holocaust genocide.
There are some cases where there have been events of anti-abortion violence in the USA outside abortion clinics that have been attacked by extreme Christians who believe only God has the right to take life. E.g In 2015 Robert Lewis Dear shot and killed three and inured nine more at the planned parenthood clinic in Colorado. He believed that the Bible condemns abortion and he believed he was doing God’s work.
Religious extremists are not to be associated with general religious groups because the overwhelming majority of the religion reject extremist views.