The Crusades was not Gods will. It was the Church claiming it as Gods will, to try to obtain wealth, power, and land. If they would have come right out and said we are conquering Israel because we want their gold most people would have been against it. The leaders at that time was just as corrupt as any other tyrant, but it was not God that compelled them to do it. It was the excuse they used just like terrorist falsely use Islam to justify their actions. Again it boils down to personal choices based upon the wants and needs of the person initiating the action. Not the religion they claim to be. People who hide behind religion or blame religion for such evil actions are only fooling themselves.
On the other hand, I would not want to reduce this solely to a question of individual responsibility. Within thought, within ideological systems- and therefore also religious systems, political systems and and philosophical systems (including atheistic philosophy), there emerges tendencies within which lay waiting to be exploited more than others, or which, followed logically, may very well result, by their own momentum, in gross violations of human dignity and freedom.
History has been checkered with this. Religion is not excused, but neither are its critics. The Twentieth Century, for example, has seen some of the worst results of secular utopianism, its goals, methods and its morality. Movements, which began with noble aims, took on a momentum of their own and, in the heat of the moment and passion for the future, seemed to abolish the individual and sweep him up into a collective act which would transform the world. Systems purported to have replaced religion were found to outdo it even in its vicious aspects. And who can ignore the great and brilliant philosophers, who, at the hands of later generations, would have seen their ideas put towards death factories?
It is not only individuals who are open to criticism, but ways of thinking as well, IMO.