Terrorist radicalization is a relatively new concept among scholars within the last decade.
As opposed to assuming these acts are a result of some foreign force, the vast majority of scholars have come to understand that becoming a terrorist is actually a process (this would explain terrorists attacking their own cities, towns, or countries---aka homegrown terrorists).  This process is called radicalization.
There are several significant sources of radicalization.  One such source is, surprisingly enough, considered to be religion.  The connection that religion has to terrorism dates back over two thousand years, so it’s hardly a new source, yet one of the most everlasting.
While it’s true that this particular source tends to be overshadowed by ethno-nationalist or ideologically motivated terrorism, it is still highly pertinent.  Groups that find religion as a source of radicalization have both overarching aims and motivations that strongly reflect a religious character or influence.  The Islamic State can be used as an example of this; this group does have an aim and motivation that reflects a religious influence.  They are an organization that represents a certain form of Sunni Islam: radical Salafi Islam.  Their goal was to establish the caliphate and allow divinely revealed Sacred Law to precede over arbitrary law.
It was then that individuals separated themselves into two categories: Muslims and infidels (the Islamic State, of course, continued this separation, declaring anyone that does not agree with their interpretations of the Quran as infidels).  Other major sources of radicalization are disillusionment and loss of belief in self efficacy, social disorganization, group grievances, marginalization/discriminatory treatment of specific groups (i.e. Muslims), unpopular policies, etc.  Social disorganization, group grievances, and marginalization are three factors that often intersect with one another that lead to radicalization.
Negative socio-economic factors such as corruption and lack of good governance can be found as the source of radicalization for many terrorist groups today.  When looking at the Islamic State, we can see examples of the aforementioned factors.  Many individuals who decided to join the organization were disillusioned with the current government; they are highly frustrated with the perceived failings of their government.  Many were without work, resources in rural communities were nothing short of dreadfully lacking, and eventually years of war ensued.  Hand-in-hand with disillusionment, several members who joined the Islamic State felt marginalized.  In both Syria and Iraq many Muslims (Sunni Muslims, to be more specific) felt as if they were underrepresented, oppressed, and that the government’s lacked concern whereas many others joined the organization in response to a feeling of religious and cultural repression from the West.  For those who lack self-efficacy, this group provided them with not only a collective identity, but a means to accomplishing what they believed they otherwise could not.  Funnily enough, the internet is now one of the most common sources of radicalization.  This provides groups with the ability to reach millions upon millions of individuals who they would have otherwise never been able to reach.  Their message, ideals, and aspirations are now able to be spread across the world in a mere instant.  People can become indoctrinated, so to speak, by the content that reaches them and thus begin to adopt the ideas of the specific terrorist organization.  The Islamic State is a major perpetrator of online propaganda; they utilized social media (Twitter, Instagram, YouTube), the Internet Archive, and developed their own social media platform to spread this message, recruit, and openly showcase their violence.  They’ve garnered worldwide attention this way, especially for their unchallenged violence; videos of beheadings have circulated, communication has opened up on a global scale, and people in foreign countries have been moved to action.
