Most terror attacks that have responsibility claimed by a group are because of some perception of being maligned by a group, a nation or a people. It's the 'West', the Jews (of which I am), the Media, the Infadels, etc. ISIS for example was mainly formed after The former Iraqi President went after and murdered numerous Sunnis in a paranoid delusion that Sunnis were out to get him and his government. He inadvertenly helped to creat the ISIS movement as a response to his actions. I highly recommend watching 'The Rise of ISIS' on Frontline, PBS for a full explanation.
To become powerful, ISIS and other terrorist movements need two components that they currently enjoy: vulnerable and impressionable youth. The ISIS Sunnis that multiplied in great numbers were well funded, had seemingly inspiring speeches to listen to, and most importantly, it became a paying job..unemployment in the region was out of control; ISIS funders clothed and fed them well. This was in contrast to the existence that they had before joining. However, there has to be an element of violence hidden in these individuals as well.
The group that went on the Parisian rampage had similar roots. They were youths that were seemingly disillusioned with their lives, susceptible to the infamous and now dead American-Arab terrorists indoctrination and funding, and inspired to revitalize the hatred of the Charlie Ebdo staff and founder. One may ask why they murdered some Muslims in their path and innocent police? Why did they randomly take over a Jewish AND Muslim patronized kosher bakery and slaughter several folks there? It seems per a phone conversation that it was a Jewish place and seemed to fit well with one of the terrorists mindset for the moment. It also seemed random for the brothers to go out in a blaze in the separate print shop that had no particular target for anti-Muslim publications.
So strip it all away and what are you left with. Middle schoolers who grow up into teens and young adults, with perhaps some psychological scars and even organic hostile tendencies. Throw in, and not evenly in all cases perhaps, poverty, violent religious indoctrination, and a lack of positive, healthy, caring purpose and you get someone who is at least susceptible to the gang environment or terrorist/violent community of causes. When it crosses the unclear border of loss of empathy as when Saddam Husseins uncle had him murder someone when he was a youth, some individuals never look back at a normal life again. Some of the ISiS 'fighters' are like a lion who has tasted human flesh and becomes a man eater, requiring sadly, to be put down because there is no return to normal behavior.
If only we can hope that world sentiment will discourage some from joining up with these groups, and if families do their best to nurture and foster peace, then we might fight these tendencies It seems it will be a long but worthy fight, to foster peace, brotherhood, sharing, compassion and goodwill against despair, helplessness and loneliness. Keep the faith and G-d bless this World.