Religion is the poison that leads people down these paths. Christianity has plenty of human blood on its hands from the past few millennia, too, and Christian terrorists exist - albeit in smaller, less organized numbers these days. Neither ideology is the solution to the other. Throw in socioeconomic conditions in the parts of the world where Western powers have their influence and it's no surprise this keeps happening. The hole just gets deeper all the time.
No god presided over Paris or Beirut today. Each side thinks their messiah is on their team while innocent people are lost in the balance. Horrible attacks like these beget retaliations by Western militaries, and the cycle repeats. Innocent people will be murdered while zealots squabble over which of their deities is real, waiting for the day their gods cleanse the planet of the "wrong people".
My heart goes out to the people in Paris and Beirut. The French are our first ally, and a large reason why we as a nation exist, and I hope we as a people remember that when France needs us in the future. Nothing can replace the lives of the people we lost today, but I hope there comes a day when the barbaric ideas that lead men and women to do this sort of thing cease to exist. Until then, E Pluribus Unum/Vive Le France/Vive la Liberte.