Though like the scum of any group, yes they do reflect poorly on the group as a whole on some level.
The problem with this is that when it comes to religious groups, it's the fanatics who have the majority of scripture on their side.
Moderates, on the other hand, have to pick and choose their beliefs, ignoring the worse parts of scripture.
Southern slave holders could quote multiple verses of the bible to back their arguments that god condoned slavery and they were right, the bible clearly supports slavery. The prohibitionists, although they claimed god as an authority for their views, didn't have the backing of the bible for their claims, they had to just make shit up.
Westboro Baptist Church also is a very literal biblical group. They ARE right, god DOES hate fags, the fucking bible says so in clear black and white letters.
The Jihadists are the same, they are the ones who are following the Quran to the strictest letter that they can. They are arguably the TRUE Muslims and the moderates ignore Quranic verses because they aren't insane psychopaths.
All Abrahamic religion is, at it's core, psychopathic non-sense. Yahweh is a tribalistic war god of the Hebrew people and is an insane tyrant. It only follows that the true followers of this god are tyrannical, insane, war-mongering and bloodthirsty.
Terrorism is the natural result of believing on bronze age, barbaric moral codes.