When Muslims do this sort of thing, not only can the site the Koran for endorsement, they can point to century upon century of accepted Islamic scholarship that VERY explicitly endorses such practices.
If a Christian does something like that, he might, maybe, depending on the situation, find some statement in the bible that arguably oks it if you look at it JUST right, but they will have ZERO Christian scholarship to support them, even if they go back to Saint Augustine and before.
When various Islamic leaders preached Jihad over the centuries, they could do so from a solid footing on the Koran, Hadiths, etc.
When Pope Urban II preached crusade he did NOT preach it with biblical argument (because there was none), he preached it from a euro CULTURAL position, and tossed in a few bible words and names.
He basically made the argument that Muslims taking the Holy land was like some foreign invader taking the land of their family, and then made the NOT biblically supported argument that God would see it that way too. Read the bible as you like, but you won't find much from Jesus on that (although a few old testament verses could probably be stretched....).