Naomi, do you distinguish between killings which are simply because of the religion and killings committed by those who are of the same religion but which are motivated by fear of loss of lands, rights or freedom , revenge for previous supposed or real injustices, or to gain control of others' lands, wealth, or people?
Few killings in the world are simply because of a person's religion itself. Catholics in Northern Ireland weren't killing Protestants because the Protestants did not follow the theology behind one branch of the faith.. It wasn't a debate over the status of the Virgin Mary or the Pope made murderous! And the same applies now in the Middle East and has throughout history. The war or killing may be dressed in religion, but the underlying reason for it has nothing to do with simple theology.
Nowadays, the West does not invoke religion. We do not invade countries or have wars excused by it. There is no Armada sent on its way with the Pope's blessing and authority, in the hope of killing a Protestant Queen for her blasphemy; of course, seizing her lands and gaining the control of her country was not behind such an enterprise ! No, we think of other justifications. We do not invade a country because its people are heathens but because its leader might in the future have weapons which he could use, and not, of course, because it is defenceless against our forces , it has lots of oil, and we'd like to have greater influence in the region.
What the holy books of any religion teach, or what its clergy or clerics teach if faithful to them, has really nothing to do with that. Power over others has.