The how and the why are the same.
Because they believe.
They believe that our culture is evil and is a threat to theirs.
They believe that their religion demands that they conquer our culture and convert us to their faith by force if necessary and by whatever means necessary.
They believe that dying in the defense of their faith is a free trip to paradise regardless of any sin they may have committed.
Such faith is a great motivator for a soldier.
If a soldier if fearless in the face of certain death, if he is willing to die to achieve his goal, how do you stop him (especially when you have no fronts and the enemy does not wear a uniform)? If you have even a small army of such dedicated well-trained warriors you can do great damage to much larger foe.