Depending who you talk to, a root of a great deal of evil or a great deal of good. Often seen as "the biggest unnatural cause of death in history" though more often than not is only used as an excuse for wars that are actually about other things - such as resources, territory, or pre-existing ethnic differences or political grievances.. unfortunately the majority are judged by the fact that the minority that twist it for their own purposes happen to be in positions of power. A definition would be a set of beliefs and/or practices that a follower of the religion adheres to, and sees as "truths". Common to most religions is the "Golden Rule" - do to others what you'd have done to yourself - unfortunately, not many "religious" people practice it
Israel/Palestine, Northern Ireland, Iraq, the majority of conflicts since the Crusades - Wars perceived as being for "religious" reasons when the truth is very different, the two sides just happen to be of different religions.
The Crusades - wars fought misguidedly for "Religious" reasons.