Crusades was a largely political alliance between the papacy and european nobles to reclaim culturally and religiously significant territories in the Middle East, most notably Jerusalem.

Of course, religious motivation was behind it, alongside racial, cultural and political motivations. The Pope had a lot to gain by urging powerful nobles and rulers to unite behind a cause of his choosing, rather than turning on him and eroding the powerbase of the Vatican, which had been happening in the centuries leading up to the first crusade.

Sectarian violence like the trouble in Northern Island between Catholics and Protestants, and in Iraq between Shi'ites and Sunnis? Their faith is a mask for the power hungry crooks that use terror and violence and profoundly non-religious means to achieve their ends. If not religion, they would soon find another motivation or ideology to rally their cause.

I'm not necessarily saying the religion is a good thing, I'm saying it's simplistic and naiive to attribute religion as the cause of these problems. And to say "religion is the no1 killer of all time through history" - that's just not the case at all when you think about it