There is not one Islam
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004:089: They desire that you should disbelieve as they have disbelieved, so that you might be (all) alike; therefore take not from among them friends until they fly (their homes) in Allah's way; but if they turn back [to their homes], then seize them and kill them wherever you find them, and take not from among them a friend or a helper.
004:091: You will find others who desire that they should be safe from you and secure from their own people; as often as they are sent back to the mischief they get thrown into it headlong; therefore if they do not withdraw from you, and (do not) offer you peace and restrain their hands, then seize them and kill them wherever you find them; and against these We have given you a clear authority.
A terrorist might interprete this one way, a different sort of Muslim might interprete his scriptures more symbolically.
You can look in any religious scripture and find justifications for about anything.
I look more at how people interprete their religion that what it seems to say literally.