Then why are only some Muslims participating in terrorist acts, and not all Muslims? What is different about Muslims living in Syria than Muslims living in Indonesia? If you want to know, the difference is their circumstances and living environment. People living in a war-torn environment where they have no prospects of a stable and safe future are thousands of times more likely to engage in violence or extremism than people living in countries where they can go to work and raise a family in peace.
Even if there was no religion, the people in some of these devastated countries would still have no prospects waiting for them if they tried to live a peaceful life. If you take this pre-existing mass of desperate people with no real hope who are already familiar with violence on a daily basis, and then mix in ambitious and ruthless demagogues who see an opportunity to grab power for themselves, it becomes a volatile concoction that starts to do real harm.
Despair, poverty, base greed, the desire for power, mistrust, resentment at distant powers that one feels are responsible for their lot in life. These are the ingredients of political violence. They are as old as human civilization, and have been true for every place on earth and every people. Islamic fundamentalism is just a convenient flag for these forces to rally around, but that flag could have been replaced by anything.
Groups like ISIS are just the most modern incarnation of an ancient socio-politcial force called warlordism. In times of chaos, those who are the most brutal rule. You can look at any point of history and find examples, from around the world. The things these groups are terrifying, but they are not unfamiliar or strange. And the forces that motivate these atrocities are clear: poverty and chaos.
If you want religious people to stop killing others in the name or their religion, you just need to give them stability, safety, and a chance to make a decent living. Once people have peace, they won't want to risk losing it by engaging in violence.
