It would be nice to get past the formalities around "every group once upon a time committed an atrocity" and focus on the problems in the present day.
Today if you are a malcontent in western society there are systems in place to detect you, discourage you, and divert you to treatment. Certainty you will not find and quasi national groups inciting you to armed violence and providing you with training, planning aid, and financing. You may slip through the cracks and become a Jason Borque or a Marc Levine, but there are good odds that instead you will be discouraged and that you will actually received support to send you in a different direction.
In the Muslim world even if you are born into a moderate family people with those sorts of violent aspirations can quickly link up with like minded malcontents, find lots of encouragement and lots of philosophical justifications for violence, as well as actually get training on weapons use and be provided with machine guns, rpg's, high explosives, and other military equipment to help you kill more effectively.
There is currently a difference. Currently the terrorist threat is primarily a Muslim terrorist threat and we ought to be able to talk about the reasons why.