Almost every genocide in history is committed as a form of retaliation or revenge. WWII was directly sparked by the unfair conditions imposed on Germany in the Treaty of Versailles. The same can be said about the genocides that occurred in Rwanda, Cambodia, Sri Lanka, and Armenia, among other countries.
Alienation is a powerful tool; think of the Columbine Massacre and all recent high school shootings. What was the root cause? The suspects/attackers all felt alienated and bullied. Now, put yourself in the common Muslim's position.
You're a 16 year old boy who feels like you don't belong anywhere. People look at you with hate, call you a monster, and condemn your entire family and culture because of the actions of a few, even if you've done nothing wrong. Then, a group of people show up who make you feel like you belong, like you have a purpose in life. Why support, protect the people who alienated you, attacked you when you can be strong and seek revenge for how you were treated? They speak the same language, include you and make you feel important, and treat you as if you are their brother. Suddenly, at 16, you're given fancy new guns and women; both of which, were out of your gasps earlier on.
Imagine coming home to your family after a long flight, only to find out about 9/11 shortly after landing. Your heart breaks for your fellow Americans but because you are Muslim and was born in a Middle Eastern country, you are escorted to the back of the airport and interrogated for hours on end You are not allowed to contact your family and when it's time for meals, your detainers give you only pork, which you can't eat. Imagine how proud you are of you son, who worked hard and was given the chance to interview at one of the best medical schools in the country. Except, when he goes to one of the interviews, the interviewer's face immediately drops as soon as he sees your son and focuses the entire interview on the Middle East instead of his qualifications. These are things that have actually happened to actual American citizens. Can you imagine how hurt they feel? Imagine being in a class full of students, except people would rather sit on the ground then sit in the two seats next to you because you are wearing a veil. All eyes are on you because the Paris attacks had just happened.
We human beings are remarkably susceptible to manipulation. The Stanford Prison Experiment and the Milgram Experiment demonstrate just how easily we can commit terrible acts against another human being if we feel forced into it or if given power.