Feeling undervalued by society
Feeling undervalued and under-appreciated by society can make a person feel like they have nowhere to turn for support and encouragement. Extremist groups give people a sense of purpose and drive in their lives that they may not have if they're feeling underrepresented in their society.
Feeling undervalued or ignored by society can lead to a myriad of violent thoughts and actions. Being undervalued by society is more of an external factor than insecurity, anger, or loneliness. The feeling of being under-appreciated can stem from socio-economic barriers or other external factors beyond the individual's control.  As people feel undervalued, they begin to look to people to blame. Blaming others can result in a loss of empathy and an increase in angry emotions. This anger can be taken out in violence against other people while turning into extremism. When people feel undervalued, they can be manipulated by extremist groups who pretend to show care and interest in that individual to recruit them.  There is evidence that this recruitment even starts from a young age, with school-age children and young adults, who may be the target of bullying or violence in their personal lives.  Extremist groups make those individuals feel valued and appreciated and feed them misinformation about the righteousness of their cause to gain support. Their value and appreciation cause the individual, who may have felt disenfranchised and undervalued before, more confident and more prone to violence on behalf of that extremist cause.
Extremists don't manipulate people for their own personal or causal gain. Believing that extremists manipulate other people into following their cause is naive. The individual chooses the path of extremism and follows it on their own for their own reasons. Being undervalued by society does not mean that an individual is too weak to resist persuasion or misinformation from extremist causes.