Radicalization is the process of taking someone and manipulating them into believing a certain set of ideals that could/can require it become their lives purpose and/or inciting violence on those who don't agree with or submit to the same ideals.
Radicalization can happen to any type of person through the use of physical, psychological, emotional, or monetary manipulation.
Propoganda
Examples of this include ISIS recruitment videos while they torture, behead, and execute Americans while conditioning the viewer to equate Americans with evil. Anwar Al Awlaki is the most successful Al Qaeda recruiter.
Inspiration
Radicalization can occur based on events that happen and motivate individuals to carry out attacks. An example of this is Waco and Ruby Ridge motivating Timothy McVeigh to carry out the Oklahoma City Bombing
Purpose/revenge
Those who seek a purpose or feel wronged by the world and seek revenge. An example of this is how Eric Harris and Dylan Klebold felt so wronged by the world and society that they carried out the columbine massacre for people to listen.