On the Crusades - Eminent Historian Trevelyan

'The Crusades were the military and religious aspect of a general urge towards the east on the reviving energies in Europe. The price that Europe brought back to from the Crusades was not the permanent liberation of the Holy Sepulcher or the potential unity of Christendom, of which the story of Crusade was one long negation. She brought back instead the finer arts and crafts, luxury , science and intellectual curiosity - everything Peter the Hermit would most have despised'  --  Eminent Historian Trevelyan

Therefore Crusade must be blamed on greedy rulers rather than the religion of Christianity which teaches love for everyone, even the enemy. One must learn to differentiate Christianity from Christendom. The Crusades had their source in Christendom not Christianity.