Holy War

Why is the phrase “Holy War” an oxymoron?
The phrase "Holy War" is in fact an oxymoron for several different reasons. Firstly the definition of Holy is sacred or worship. The definition of war is an act of violence that can harm or hurt people. But both of the words together means, "a war that defends or acts for a certain religion." Aren't religions meant to teach peace in their teachings? Then why do they fight? If religions believe in peace and tell fellow followers to not kill, to not commence in violent activities, to stay clear from harming other people then why in heavens name would you find this war justified. No war has any justification and to say that a war was for religious reasons just insults the whole religion in my opinion.This is why the phrase "Holy War" is an oxymoron.
Why is it that so many conflicts in the world today are based upon religious differences yet all major world religions condemn killing?
I think that many people have been brainwashed by a higher power to believe that war is a necessary resource and if it is in Gods name then it is justified even though you may kill many other people. Also in many religions if you can convert someone into your religion most of your bad deeds become cleared off and you then are justified in the desperate measures such as war.