Having lived in India as well as Saudi Arabia and then moving to the west as a Muslim, I can answer that. And answer that is all I will do, it is not a justification for the actions in any way or form.
The answer is short, education. There is a huge populace of uneducated muslim's in the world, probably the highest percentage of the 3 major religons in the world. With education comes tolerance and understanding that so many lack. Since the 1960s, Saudi Arabia started preaching its brand of wahabism to a lot of muslims, not only at home but abroad. This gave you the likes of Taliban and extremist Islam was born. Afghans were know for revenge long before Islamist Islam took roots with Saudi Sponsored madarsas to drive away Russians (I will let you guess who thought it was a good idea to drive away Russians in this manner). All these uneducated fighters were given a cause to fight in the name of the religion.
Living in Saudi Arabia, one thing that is quite apparent is that they are very strict in terms of what they intepret Islam. Their religious police would come out at us with sticks if we dared play soccer during prayer time. This brand of Islam was exported with oil money to a lot of places and you get terrorism.
Not only that, a few corrupt indviduals will go murder innocent non-muslims in the name of Islam. The same breed of uneducated non-muslims would then go kill muslims and decades of enimosity lasts between societies and cultures. You get into a feedback look of hate and suffering exploding to sad events like 911.
IMO, this generation of terrorist cannot be enlightened, the focus must be to bring the new generation up with education and tolerance. Taliban know this full well and you get 14 year old girls shot, only if the west would realise this too.