Well Clifford, as an atheist, I'm going to suggest it's "people, acting in the name of religion".
Keeping in mind I believe I said, "throughout the ages", here's a few highlights: The Crusades, The Spanish Inquisition, a multitude of religious and ethnic purges, the crucifixion of Jesus of Nazareth, and in recent times, the destruction of the World Trade Center.
It's easy for both of us to get bogged in a "which came first, the chicken or the egg" conundrum.
But the truth is, the human species can't live up to religious tenets, and very well may distort and scapegoat its concepts for selfish purposes. Remember, the US was formed, (specifically "New England"), primarily by the Pilgrims being driven out of England for lack of adherence to the tenets of the Church of England. Did they fall, or were they pushed, the egg asked the chicken.
Anyway, here's Loreena McKennitt with her truly elegant musings on the topic: