" ...the clerics usually don't do anything but give justification for things the killers already want to do."

The question of how far "usually" extends makes it difficult to determine just how right or wrong that opinion is. But I submit that in a very significant number of instances, the things that killers want to do are largely determined by the fact that they have been indoctrinated from birth to accept what they hear from the clerics of their particular faith with little or no critical thought.