Religion. 

Any vague poetry which is considered holy, absolute, unchanging truth, beyond criticism, a sacred guide to living life may eventually lead to terrorism.

A man needs some sort of identity, he craves it. Apparently being a human is not enough. 

He wants to be a part of an association or an organization, so he can be a proud member or a leader. 

Throw in God, superstitions, medieval fallacious logic, racism, misogyny, and so on. 

He is ready to die or kill for it for his prideful identity.

A voter of a nationalistic religious/ethnic party thinks he, himself, or an extension of himself is in power. 

He would participate in the propaganda and warmongering against people of other identities. Or turn a blind eye to it, and even become sympathetic to certain crimes for a 'good' cause.

It matters not what religion teaches, it gets militarized under a theme of protecting it. 

And then there are perceived threats and offenses with schizophrenic vibes.

He wants to contribute to his religion and nationality etc, to earn even more pride. 

But religion is a vast vague wasteland open to interpretation. Because telling big lies such as there is a God isn't easy. 

One easy way to contribute to it is by participating in extremism. 

Mundane elements such as being a vegetarian get radicalized into a compelling need for it to impose on others. 

But not the real essence of religion, which is being a good human, is lost somewhere in the ways employed to fulfill it. 

Blinding pride ensures that hate becomes more and more violent as you criticize it. Still, we do criticize a bad ideology as rational beings should.