ANSWER: It is important to make a distinction between the first crusades and the ones that followed after.
The crusaders, who had taken up arms for the deliverance of Jerusalem, suffer themselves to be seduced by the wealth of Greece, and stop short to undertake the conquest of Constantinople. From that time the spirit of the Crusaders begins to change; whilst a small number of Christians still shed their blood for the deliverance of the tomb of Jesus Christ, the princes and the knights are deaf to everything but the voice of ambition. The popes complete the corruption of the true spirit of the Crusaders, by urging them on, by their preaching, against other Christian people, and against their own personal enemies. The holy wars then degenerate into civil wars, in which both religion and humanity are outraged.
(History of the Crusades, Michaud, Vol. 1, Intro)