I see your interest Nailit, and to quote:
Radicalization (or radicalisation) is a process by which an individual or group comes to adopt increasingly extreme political, social, or religious ideals and aspirations that (1) reject or undermine the status quo or (2) reject and/or undermine contemporary ideas and expressions of freedom of choice.
The above definition of 'radicalisation' happens in both examples that you have given to a certain extent. However, as the definition points out in exert: 'adopt increasingly' - most of the cults you mention are stable and do not 'increase' in their religious views. Therefore cults are not often linked to the word 'radicalisation'. 'Radical' is a change, stable is non-change (or very little)