Those are extremists that believe their religion is above everybody else and they will kill and die for their god. Reasoning with them is not applicable anymore. They are already willing to die for their religion, and they'll do anything to prove it. The only way to even get close to stopping them is eliminating them. But there will always be terrorism in the world. As long as there's ignorance, there will be terrorism. As long as there's humans on earth, there will be ignorance.

There was also the Crusade, where people literally died so they could have their religion.

Killing for religion is morally and legally wrong. If you have to kill somebody to have your religion, or you kill somebody because your "god" says to, things need to change. Nobody should have to kill or get killed because of their religion. Set aside the differences. Let them be.

It was different in the crusades. Both sides were fighting to perserve their way of life. It wasn't illegal, it was war.

War caused by two seperate belief systems that led them to kill eachother because of what they believe. It's wrong.