Oxymoron

The phrase "Holy War" is an oxymoron because war is usually not considered as a holy act. If you're religion prevents you to kill someone, but whenever you have war you do kill someone, technically you're not following you're religion. This is not always a bad thing, if you kill some one just because you want to and you're holy book states that it is a crime to kill a person, then, yes you've committed a crime, and it is considered a terrible act. However, if you kill someone with a decent reason, like the man is a terrorist, even if you're book tells you not to, it wouldn't be considered that bad. Also if you're in war, then its also not considered a bad thing because you're fighting for you're country.

Source: http://www.inxart.com/images/mideast/inx_040302d_kozlowski.jpg